Okay, I just actually read my own review and noticed it's quite a lengthy piece of work. If you are actually considering buying this game, I'd recommend actually reading this (and other!) review(s) that are of decent length because this game is not easily captured in just a few words.
Mount & Blade: Warband is one of those slightly niche games out there that captures the heart of gamers looking for a medieval world where you play as just another soldier able to pave his legend or die forgotten. There is no storyline, no heroic destiny - you are not Dovahkiin, the fate of the world surprisingly does not rely on your actions, and no, you don't get a magical sword that will vanquish all enemies. Now some people might find this to be a deterrent, but for a game entertaining as Mount & Blade: Warband, I only saw endless replay value. Having put thousands of hours into both the original Mount & Blade as well as Warband (both of which I originally did not buy on Steam) I can assure you that if this game fits the genre of your preference, you can play it endlessly.
Now what sort of genre IS M&B Warband? You have a lot of choices, you can be a solo mercenary (which is very very hard), lead a group of mercenaries for a noble/king, swear fealty to a king of a faction, or attempt to become the king of your own faction. Remember, M&B Warband is a sandbox game in which you can choose what you want to become. Whatever story you feel like making, embrace it and become. But, for the most part, it is a first-person medieval combat-based campaign of a character trying to make his or her way in the world. Yes, there are some tactics involved, you can create your own little trade caravan and take over the market, you can go around helping all the peasants in the world, and if you have a masochistic streak, you can attempt to work your way into the politics of the game (which like politics, is frustrating). Yet, in the end, it all comes down to fighting your way through life.
Let me jump right into my favorite part: the combat. M&B has a very unique combat system (there are a few glitches here or there, but nothing game-breaking) involving first-person medieval combat on a battlefield that quite possibly may be strewn with horses, people, corpses, quite a few arrows and crossbow bolts (mostly flying at you). You have a variety of weapons to choose, from a heavy siege crossbow to a throwing axe to a mere wooden staff to a glaive to a two-handed sword to... you get the point. Add all of this to being mounted on horses (which have differing physical attributes) for mounted combat and you have the chaos that constitutes a true battlefield.
There are also sieges in this game, which allow you to actively use the terrain to your advantage... or disadvantage. It is very hard to siege a castle on the hardest difficulties (because I mean, when they are sitting on walls raining arrows on you...) but also very rewarding as you can then claim the castle and its revenue for your own. My biggest issue with sieges is that for most cities/castles, you are given a single ladder/siege tower with which to conduct your siege (infrequently you get 2, of which your/their troops only use 1). No matter on which side, this makes for siege defense quite practical when actually participating as every man on the wall is worth a hundred below it. (The combat simulator, which is activated when you don't feel like participating in the battle yourself, generally is very different from actual combat and tends to make defenders weaker than they actually are. At the same time, your troops will also not be getting orders from you and thus will be strategically weaker as well.)
The hardest part about the combat system for myself and I'd believe most others was the ranged combat - especially mounted ranged combat. Everything short of wind and air resistance is factored in, from flight time to the drop from gravity to the sway of your horse to the angle of release to the point of impact on the enemy. It is challenging, rewarding, and all together epic. I'd like to say more about the combat, but if I let my thoughts roam wild on this game we may be forced to deal with a book instead of a review. So let me leave you with an image: Picture yourself holding a shortbow, moving at a slow gallop on your horse, faced with fifty knights charging you with lances couched.
Moving on to the AI in this game: The enemy AI is pretty good (if you play on harder difficulties) and their characters are very strong - as strong as if not stronger than you. You also have the ability to deal with merchants, caravans, kings, nobles, ladies, and the odd "adventurer/hero" who may be willing to join you. Unlike the original M&B, Warband also offers you the option of recruiting vassals from other factions into your faction if you choose to start one. This has made attempting to create a kingdom drastically easier since you no longer need to defend your territory alone from the might of five other kingdoms and their thousands upon thousands of troops. The game also factors in your relationship to the NPCs, which can get very messy very quickly (this is something you should definitely experience for yourself, as words don't really do it justice).
There are also a vast variety of mods for this game (though the vanilla version in and of itself is endlessly playable) that create various scenarios/settings in addition to the vanilla version. Furthermore, Warband brings online multiplayer which (though I personally did not partake in much of) enhances the experience even more.
Warband is unique. It is niche. It is epic. It is the perfect game for a gamer who wants to live his/her own story instead of a repetitive tale created by someone else.
I hope this review helps with your decision making. If you have any questions/concerns or would simply like more information, feel free to leave a comment or contact me.