The build of the game I got to play was alpha, although it was pretty slick and I didn't experience any issues. Ubisoft tell me that there will be a closed beta coming up later in the year before the game gets a final release. Personally I can't wait as it was a lot of fun.
Overall if you are a fan of Halo then you have to get this game. And if you are new to Halo then you won't be disappointed either. Go on, get it, now. Halo 4 is available right now for the Xbox 360 - and frankly it's a must have game. It's not just one of the best games of the year, it is THE game of the year.