It's great fun. There's an active modding community, the base game is excellent (Warband is basically an expansion and some clean up, not a sequel per se), and the world is very much interactive.

As for the game play, what makes Mount and Blade is the combat. In the early game it's generally your character and maybe a small retinue fighting bandits, doing tournaments, maybe doing some mercenary work, etc. Eventually you can join up with a faction, join a rebellion against a faction, or make a bid for taking over by yourself (technically you can try that last one from the very start, but that's inadvisable). Beyond that there's the social/political systems involving honor, relations with individual nobles, etc, which ties into the warring - nobles who favor you are much easier to get on your side, enemies made will go after you specifically, so on and so forth. Later in the game, the bigger battles start to appear, sieges become a very big deal, you might get into non-tournament duels, and bandit killing is generally not worth the time (except for when you're just in the mood to murder some bandits).

Warband also has a demo, which gets you to level 7. That generally only covers the early game, but it does at least provide you with enough time to get some grasp of the basic mechanics and whether you like them.