This game seems to be somewhat controversial due to being a standalone expansion to a previous game. I bought it without having played the previous installments, and I find it awesome. I have tried several of the latest fantasy 4X games in the spirit of Master of Magic (Age of Wonders III, Warlock: Master of the Arcane) and this one is by far the best IMHO.
The balance between strategy, city development, hero development, tactical combat and adventure/RPG events is just right. The magic is great. The tactical combat is good - not as deep as in AOW3, but more than enough in my book. Very similar to that in Master of Magic. The variety in sovereigns and the possibilities for roleplaying ensure replayability.
But the best feature is that you can create your own units! Instead of having only a predefined range of units, you get some guys, give them a particular weapon, armor, mount, special boni and characteristics, art, etc. and you design your own kind of troop - of course the more powerful you make it, the more it will cost to actually build one. I spent hours fiddling with the unit builder alone.
The AI has been criticized, maybe it's not the best but to be honest I don't see the difference with other complex 4X games. Even in an universally appraised franchise like Civ games, the AI is just not that intelligent and higher difficulty levels work by cheating, giving the AI boni and knowledge that the player doesn't have access to (those nuclears that always went to your only city without SDI in Civ 2, lol). I suppose adding extra elements like magic doesn't help with making human-like AIs - the AI in Master of Magic was notorious for doing really dumb things and cheating a lot, even more so than in Civ, and all of us that are old enough to have played that in its time had a great time with it. I'm having a great time with this one too, and to be honest the AI didn't catch my attention for being bad at all, I only learned people had a problem with it by reading reviews. Maybe it was worse in previous versions and they have improved it over time, as I bought the game when the latest version was already 1.7.
My only gripe with this game is that it doesn't have a hall of fame: even though it gives you a score and a rank, it doesn't keep track of your best scores... it's a somewhat dumb thing to care about, I know, but I'm competitive and seeing my best scores and trying to beat them motivates me a lot. The developers said they are going to add this feature eventually, I hope they do.
But in summary, great game, lots of good content and many hours of fun in it. I think it's very underrated and it's a pity that such a gem is not better known, that's why I have decided to write this.