I've spent more time playing Mount & Blade and Warband than Mass Effect 2 and Dragon Age put together; it's that fun
Now I should elaborate here that when I say better, I'm not referring to graphics or presentation or story or sound or any other technical aspect of games. Certainly, in almost all of these areas, M&B:W simply doesn't compare; as has been mentioned in several reviews thus far, graphically it looks like it could have been produced 10 years ago. However, when I look at comparing games and saying which is better, I mean simply 'more fun'. Roughly 120 hours of Mount & Blade and its stand-alone expansion, Warband, and I feel quite qualified to say that M&B:W is the most fun game I have played, and the second most addictive (World of Warcraft takes the crown there).
The game is essentially divided into two parts. First there's the strategy side, where you ride around the game map with your army visiting or besieging towns, buying armor and weapons, collecting quests, recruiting companions and soldiers, or terrorising villages. The fact that these are just a small number of the possible actions for you to take displays just how wide-ranging the gameplay of M&B:W is.
In the aforementioned 120 hours of play that I've had, there are still a number of major aspects of the game that I just haven't touched that would likely be the the main reason for playing for other players (that being the trade system, rightful ruler quests and ruling your own kingdom).
There's so many different enjoyable activities to occupy yourself with in M&B:W that the fun will stick around for many, many weeks.
Of course, the strategy side isn't even M&B:W's strongest part. The other half of the game is more familiar of typical RPGs such as Oblivion or Gothic, where you control your hero on a battlefield.
The best aspect of this part of the game is just how much freedom and flexibility there is in the combat system. There are no annoying, long-winded animations that you get stuck in, nor do you have the turning circle of a walrus. If you want to do something, the game essentially lets you.
You'll be surprised at how many times something incredible and unexpected happens that makes you sit back and say 'wow'. Playing while with friends on vent lead to myself making comments every few minutes:
"I was getting charged by this knight on horseback, so I started moving right, then at the last second dodged left, jumped, and hacked him right off his horse with my two-handed axe as he road past! Then I took his horse and rode off!"
"I had an enemy infantry lined up with my bow, but just as I released the shot one of my own cavalry rode into the shot. My arrow went through the horse's head, killed it instantly, the guy was knocked off and swiftly killed by the enemy infantry. Woops."
"I just had an awesome duel with the enemy King in the middle of a battle! We were going at it, both getting hits in, both covered in blood, then I finally took him out with a feint slash turning into a thrust... Then an arrow came out of no-where and finished me off"
"I had a massive battle on an open field with rolling hills. I set my archers up the top of the hill, infantry just in front of them, and had my cavalry follow me. The enemy charged the archers and their numbers were halved when they eventually reached the protecting infantry. Then I took the cavalry and charged them from the back, killed all of them, didn't lose a single man"
It goes on. Suffice to say everyone who listened to these stories bought the game themselves and proceeded to tell their own unique, awesome stories.
The only drawback of Warband, discounting production values (which don't detract from the gameplay anyway in my opinion), is that it doesn't add a massive amount to the original. Multiplayer is typical, player versus player fun, complete the familiar rush and frustration and elation that go with it. Single player adds a new faction and some extra gameplay mechanics, but for the most but it is the same game.
If you have Mount and Blade, I'd still recommend buying Warband, especially for the mods that will be coming out for it. And if you don't have Mount and Blade, seriously, get it. Now.