For those new to the Total War series, the basic premise of these games is two-fold: a) conquer a strategic map with countries, castles, and armies and b) fight real-time battles with those armies (and castles/town sieges). MTW2 takes place, not surprisingly, during medieval times. The focus is on Europe and nearby countries. There's a lot of great info out there on the MTW2 and TW in general, so I won't spend a lot of time on that.
Instead, I'll say that as a veteran of other Total War games, I found this version offered relatively little that was new. Especially if you've played Rome TW. The first time I played this game it was on Hard/Hard. After that it was Very Hard/Very Hard no reloading and I never lost. So the challenge level isn't there if you are good at the games. It's really a race to expand to roughly 8-10 countries, and then after that the AI is never smart enough to launch a big enough attack that will threaten your kingdom unless you leave it open to attack. The different factions within the game are interesting, as they all have some unique strengths and units. The expansion packs follow that theme. The Americas was my favorite expansion pack. Crusades was fairly boring, Britannia was very similar, and Teutonic was much of the same. You do get good value in terms of single-player game time, but it's at the expense of deeper challenges as the AI is quite predictable and not nearly strategically ruthless enough. Diplomacy is slightly improved, but not enough to make it a strong solution in the game (it's Total War after all).
All in all then this is either a very fun introduction to the TW series, or it's an amusing, but not amazing, continuation of the series.