I'm just reposting a comment I have put in different threads. Some of this might already be apparent so I apologize in advance.
I strongly urge everyone to pick up Mount and Blade: Warband. Here's why:
This game, while it's graphically low-end, is extremely innovative and fun when it comes to combat. The controls take some time to get used to, but once you do it just feels right.
It's extremely addicting once you get into it. The open-world nature of the game really lets you forge your own path. Do you want to become a wealthy merchant and rule the country with your capital? You can do that. Do you want to be a lowly soldier under the marshal of a mighty army? You can do that. Do you want to rule the world? You can do that. Do you want to be a vagrant bandit, robbing caravans and murdering farmers with your band of rogues? You can do that.
The game is easy to learn but hard to master. It rewards practice tremendously, and as you get better and better the game becomes exponentially more enjoyable.
It is ridiculously moddable and has an extremely active community based around mods. You will find any kind of mod out there updating the graphics, giving it a total conversion, or (as many others have said) putting you right in the middle of Westeros, in the popular Clash of Kings mod.
The developer, TaleWorlds, is a small Turkish company that has been dedicated and helpful in making the best gaming experience they can. This includes continuous updates as well as a great connection to the community. Currently they're developing a sequel to Warband called Bannerlord which will incorporate certain elements from fan-created mods to make it even better! But if you appreciate small developers show your support by picking up this game while you can at this price.
Warband is a great game and I always have a great time whenever I play it. Multiplayer is very active and if you like, you can pick up the Napoleonic Wars DLC. People generally shun the With Fire and Sword DLC because it was made by a different developer and takes a lot of the charm out of the game. I haven't played it myself, though, so I can only speak for its reputation.
Finally, pick up the original Mount and Blade if you really want, but do not feel the need to do so. Warband is essentially an updated, expanded version of the original and it's better across the board--but again, it's up to you--the original is still fun in its own way.
There is a supportive community over at /r/mountandblade, so check them out!