First off, if you are good at single player strategy games, the single player will be of little challenge to you here. You'll have to play play-by-email multiplayer games (or find skirmish games online). Play-by-email games will last quite some time (months) so be prepared for such things. I'm not a huge fan of this but I got used to it. However, if you don't have the patient for such things, it would be in your best interest to stay away.
Now, on to what I really dislike.
This game has so much depth to it that, as others have put, the strategies are endless. However, the mechanics of this game are nowhere near the level they need to be. To put it simply, you lack sufficient control over your units and will find a good part of the game will be to just get your units (especially mages) to simply do what you want them to do... and then get frustrated when they do something stupid.
You see, the battles are totally out of your control. You give your units orders and then watch the battles unfold in the next turn. You can give your mages, and units, commands but they are very limited. In the early game, this isn't a major issue. But, in the mid to late game, where mages become very important, the fact that you lack good control over mages becomes a huge issue.
You can only give commanders 5 orders total. After that, the unit is totally under AI control. Not only that but, if the unit can't perform what you want it to, they may do something reckless or otherwise stupid - sometimes even costing you the unit or gems (which you use to cast spells, more or less). When multiple units do such things, they can even cost you the battle. So, a huge part of the game becomes simply getting your units to do what you want them to. Does this sound interesting to you? It definitely was not fun for me - especially when you invest a good amount of time (late game, when your armies become gigantic - even an hour or more) programming your units to only find them not do what you want. There's not even a simple command for a mage to cast one spell and do it continuously. Or to not cast certain spells.
I could go on about other AI issues that annoy me to know end but I'll end it there.
My next major gripe is the way that Super Combatants, so called SCs, work in this game. Once you get into the later part of the game, you can start creating powerful items for these large, powerful units. The problem is, they become so powerful that they can take on entire armies! Even an army composed of high level summonable units which, from my point of view, should be able to stand up to them! Swarming will not work even in the least bit! So, you must rely on mages. However, these SCs will have items that are resistant to certain magic types. So, only certain magic spells will really work on them.
But, wait, didn't I just say that you have little control over your mages and that they are very difficult to program to get to do what you want? Now you see where further frustration comes in.
Also, in regards to strategies with these SCs, they will absolutely destroy entire armies but, get the right spells in, and you may take them out before they can do anything. One good spell, with some luck, can take out an SC with one hit or even charm it to your side.
To sum it up, you may get strategies that work like this:
Strategy A >>>>> Strategy B
Strategy C >>>>> Strategy A
This can be to the point that, if you aren't adequately prepared to counter something, you may have little or, even, no way of countering it. It also means that these SCs are very important to get and all players aim to get them later in the game. Without them, you are at a major disadvantage.
So, to sum this part up, balance is not in this games best interest.
Finally, I hate the ability that units, and armies, can effortless warp around the map. It destroys the whole idea of fronts, which all strategy games have. What fun is it when a unit, or army, can appear anywhere on the map at any time? There's no way of countering this as you can't predict where they will appear; you can only guess.
If the above things do not bother you and you do not mind lacking control over your units, you will most likely find this game to be a great one. However, if you are like me, you will find this game to be an incredibly frustrating experience. I played a good five or six multiplayer games (remember they take months to complete) and I will say that my most recent one will definitely be my last.