Medieval II is a good game, but not a great game.
There are some issues with pathfinding. Sometimes when your defending a city the enemy just won't attack, even if they're the ones that initiated the battle. Half the time I just go out of the castle and meet the enemy to hurry things along.
I have a few problems with my archers when they're up on the walls not firing as well. They just watch the enemy attacking the gate, or the walls, or whatever, and I try and get them to attack... but nope. I assume this is some issue with pathfinding again, because sometimes if I move them just a little to one side, they will begin firing, very odd.
I find that taking over towns/castles is a bit easy as well. You can usually bait the enemy out and just flatten them without too many losses.
After awhile the battles become a bit boring. You really don't lose unless you have significantly less troops, or you have a bad mixture of troops. But handing battles over to your AI generals can often mean losing a town you're trying to defend so anytime one of your towns is attacked by a decent force you need to go in and actually do those battles yourself if you don't want to risk losing them. This becomes particularly tiresome when you have a large number of cities and are at war with half a dozen factions or more.
The turn based aspect to the game I find to be extremely dull. There is little to no strategy involved in this. Building your towns is repititive, you're just basically building exactly the same things over and over again, maybe in a slightly different order.
Diplomacy is very bleak as well. There isn't a lot to it. I know the game is "Total War", but I wish they'd put a bit more effort into this side of things.
Trade and Economy are equally as bare. It's really just "going through the motions", conquering is the only real way to expand your economy.
The Turn based side of things really just feels like filler, between the RTS battles and a chance to move your armies. What I would really like is if Total war games were a bit more like Civ 4 during the turn based side of things. Then you really would have an awesome game.
It's still definately a good game and worth getting. I'd recommend the Gold Edition as you get Kingdoms as well.
Definately check out the mods, if you are into that sort of thing. There are some really impressive ones out there that add a lot to the game and address some of the issues mentioned above, at least to a degree.
This is still one of my favourite franchises, but I just wish the gameplay had come along a little further since Rome, mostly the turnbased side of it, I think the RTS bits are pretty good and if you want more realism there are mods that cater to that.