As an expansion it would be everything one could want but as a full priced game it is daylight robbery.
The much touted graphics overhaul is I the first thing one would notice playing the game however it is still no where near up to the level of what we would have expected from a full priced game even 3 or 4 years ago.
The multiplayer I think is the games great strength as nothing exists on the market anything like it. It is well balanced (If one plays to their strengths) and it provides for many different play styles. All the match types feel well thought out and cater to many different strategies and tactics. The battles against the AI translate seamlessly to a multiplayer forum and I think with time, could develop a very strong community and more interesting matches.
Like the first one, the signature lack of a story leaves the player to do anything but it does mean that for the first 5 or so hours of play time you will spend a large amount of your time wandering around looking for something to do that will not kill you.
The other feature that I and many other people were looking foreword to was the kingdom management however it is unlikely that anyone who was not an avid fan of the first game and thus knows all the tricks, will even get to see this content due to the length of the game. If one goes into Warband with the idea that they may get 10 or 20 hours of play time out of it and see everything there is to see then they will be severely disappointed. The closest thing I can think of to the Mount & Blade experience would be working on a character in one of those time consuming MMORPG's.
In short, if you bought the previous game, do not bother with Warband unless you are willing to pay $35 for added multiplayer and graphics. You can pick up countless mods that will allow you to become king, mary and do all the extra things one can do in Warband at a fraction of the cost.
However, if you did not play the first game (or pay for it) then I can't recommend this game enough. It is unique and will love all the many hours you will undoubtably be playing it.