I got asked this in comments and Batreps seem too 'hard' in present circumstances, so I thought I write this while waiting for lunch!
A couple of years ago, I gave FOG a 'serious' try. If you every play FOG, you will be aware that there is no other kind of involvement possible. I had long been interested in ancients gaming and had previously dabbled via getting a Splintered Light Miniatures Romano-British DBA army. Just by the way, these are the most impressive 15mm miniatures I have seen, I highly recommend them for the periods they cover. I found that pretty fun and even took part in the inaugural Conquest 15mm DBA 'event', which was Craig C, Simon and myself battling it out in the corner in between FOW. I really enjoyed that game system. It allowed a quick game and seemed a good way to model troop types across the huge 3500 BCE - 1500ish CE that 'Ancients' as a period traditionally deals with. In the end, it fell away, since none of the other Ancients gamers at the Cavs were keen on it, all having played and discarded it. It is popular in Christchurch, but those players move in different circles to me - and didn't even make it to Conquest 2008 DBA, slackers!
So when FOG came out, with all the attendant buzz, I was keen to give it a go. After allowing it a year to get bedded in and release the list I wanted. I took the plunge in 2009 by expanding my Romano-British out via Old Glory 15s (also nice, for their price) and starting playing vs a few guys at the Cavs. My hope was that this would be an Ancients equivalent to Flames of War (FOW) for me.
My take on the game overall was that it was, like DBA, well designed and had a good consistent approach to modelling units form the huge period that had to be covered. The book itself was well-written (some disagree, but I had few issues when starting fresh) with clear tables and diagrams and of course had the advantage of the nice Osprey artwork. The army books I was less impressed with. In fact, I considered them very poor value. What I liked about the system was that a real effort had been devoted both to making the game sufficiently complex to allow a wide range of strategies and also clearly enough written (though, in the end, the grognards picked holes) that the intent was clear. In the games I played, I only had to make use of the forums a couple of times to resolve something that I was unsure about. Overall, the system impressed me and I enjoyed pretty much everything about it 'out of game'. My sole quibble with it as a game was that the 'armoured' POA seemed relatively too valuable and that the Arthurian list was a little bit bland. So I switched to Late Romans (Dominate), who had armoured troops aplenty, and plenty of variety.
'In game' was a different story though. It is odd, but I actually had more fun playing games while learning and not fully understanding fully the interactions and choices permitted by the rules. Once I did, I discovered that this game invariably left me with a headache. In my defense, I was always playing in the evenings after almost a week's work (of a mental variety), but you could not just ignore this stuff and even the game-engine proved to require quite a lot of calculating on your part at every phase. So, to be honest, I just got sick of playing it. Part of what had got me into playing it was that it was the mainstream 15mm set in this city and that would allow pickup-games, events etc. However, the standard game-size was 800pts, and 3.5-4 hour games. Events would only have two games per day. I found the game practicable at 600pts, but all anyone ever had to say to me was "that is good to learn the ropes but you need to paint up a 'real' force." So, perhaps I was in the end not serious enough!
I often see online postings about this game where the gist is: "It is a very well designed rules-set, but in the end I just did not find it fun." That is my feeling exactly. The local game store owner commented on the FOG players at events: "you look over at those guys and they are just not having fun." I think that if what you find most satisfying about a wargame is a properly balanced and complex rules-system where you can do well through taking the time to master it - FOG is the right system to allow you to match wits with like-minded gamers. I think the rules-writers knew their market and created the correct set of rules for it. I think this is a narrow market, but that need not be a bad thing.
So I was left without an Ancients rules-system once more and started playing Flames of War heavily again. Flames of War suits me well: not too much fiddly detail in the actual game play, well designed and balanced lists, rewards positive play, and the person who wins is almost always (I don't blame Martin for losing us that team-game through rolling 5 successive 1s for coy, then battalion morale, honest) the one who made better choices. This meant of course that my 15mm Romano-British/Late Roman army languished in its army-box.
In the meantime, my frequent opponent Jason decided that 28mm Warlords games stuff was too good a deal to pass up. I had long had my eye on Warlord Pike and Shotte Cuirassiers, so it was Remans vs Empire using WHFB, on Jason's recommendation. I hated WHFB. It is unbalanced, needlessly fiddly in some areas and lacking in detail in other neccessary areas, arbitrary, and there is far too much variance in some key abilities. Jason and the LGS owner kept stating that this edition had a lot of improvments over the old, but I guess that means it must have been coming from a very low baseline. So we gave Mantic's Kings of War a try. I actually do like this one. It knows its purpose: fast, fun games with lots of figs and good balance between lists. The lists themselves are pretty basic so it is more about how you move the army as a whole. The lists are not that detailed, but I think that this is a reasonable design decision. However, Jason kept wanting to adding extra rules to model Roman Pila, Pike etc etc and that 15mm army still looked accusingly at me from the shelf.
Craig has long been a 'bad influence' (this blog, for one thing, is his fault) and so I decided to give Basic Impetus a go, on his recommendation. This would offer something not much more involved than Kings of War, with the advantage of a bit more detail in the armylists. It also held the promise that one-day soon I might give some of the other like-minded 15mm Ancients players some games with my 15mm Romano-British.
My reaction after first playing was "where has this system been all my life?" It was fun, choices mattered, it was balanced, it rewarded positive play, there was none of this silly attempt to make game flow match the rules-authors' take on 'real life'. Instead, the overall outcome of the rules was to reward tactics that matched (my admittedly limited) knowledge of the forces involved - without forcing me to do so.
Better yet, there was a fully fledged version. My take on full Impetus is that "It is a very well designed and balanced game... and it is fun to play too." It adds in more detailed command and control, as well as unit reactions out of phase, but does not seem to take longer to play. I have played the full version many times now, and it is fair to say that the guy I play it against the most is even more of a fan than I am.
When doing the Scots vs Roman fantasy match up, success came through getting the small unit combined arms correct. I am now figuring out how to achieve this with Roman units. Jason plays a war band list, which has to rush forward but actually still allows you to do well if you plan right and take the initiative. There is a lot of depth in this game and I never feel that certain things dominate. Since the combined arms tactics are modelled so well, it is never rock-paper-scissors (which is sometimes the case with FOG and FOW) and unlike FOG, when the armies hit things get unpredictable and you need to start improvising. What I like about Impetus is that I first set up my army right and knowing how it should work together, then hit the enemy and scramble to (hopefully) a messy win through holding things together just that little bit better than my opponent did. So it rewards careful thought and those little in-game choices matter. However, the actual calculations are far more simple than FOG - probably due to the fact that you do not often compare factors, simply roll dice based on your own factors and sometimes compare the outcome to see who did better. I played the first couple of times with a mate who had never played any ancients before, and by game two we had the key unit stats memorised and could play without reference to anything but the firing table.
FOG was a system where my opponent would often wander off for a beer while I did my movement phase, since there was nothing for him to do. I seldom would do this myself, since some of my opponents did not seem to know all the rules well! We would often get to the end of games and call it. There would be plenty to play out but FOG is just so predictable and, frankly, tedious to play, that it seemed too much effort to finish it as the night wore on. Once I settled on Late Romans I did ok at that game, but it was a lot of work.
The quote about Impetus that I see online is that "Impetus is the type of game you will still have fun playing, even when you are losing."
Apologies that this is in rambling narrative form (I am going to blame the earthquake, since it is handy), but I think that best captures why I like Impetus and sold off my copy of FOG, despite usually liking to keep my old rules (I still have 1st edition FOW, for instance).
Addendum (ie extra thing at the end): the other system I play is WOTR and it is worth doing that comparison too. There is a fantasy and LOTR mod for Impetus but I still very much enjoy GW's WOTR system. I think it does its job - Individual heroes surrounded by a largely undifferentiated mass of troops, in an action-packed large scale game with monsters and magic included - well. I don't think that WOTR is good as a base for other genres but it does what it sets out to do and does it with flair. I do sometimes look at Impetus LOTR and think how cool it would be to get all that combined arms etc from the Impetus engine, but I also see the other elements that I would be giving up. Using FOG for WOTR would of course destroy all the fun you get currently get with the genre.