It's reasonably cool. I think the hating is because there's no tutorial. The game is very confusing, and if you ignore the buying and strategy options, it's just a lame third-person shooter with CTF elements. If you do take advantage of those, it can get very engrossing, especially with big teams.
I've also run into some lag issues, although it's only gotten really bad once or twice in ~20 games. And, like others mentioned, it can crash your 360, which happened to me once. A lot of the appeal of the full game is going to depend on whether they can get these issues fixed.
I'd call this one a "wait until people start selling it used and cheap on eBay" game, unless the reviews suddenly reveal amazing things the demo doesn't.
-Erwos