Beta's pretty tight so far (XBox 360 version here) - yeah, kind of a mix between Halo and Borderlands. A lot of similarities with mobs and weapons to Halo - swirelly homing bullets, weak-ass minions with a stronger "captain" ... but I like it. The "feel" of the game is a lot better than Halo and I really love the scenery. Lots of cool vistas to explore.
I am very curious to see how robust their servers are when day one hits. Right now it's pretty sweet with only a few hundred/thousand people using the game but when it's half a million or whatever (this game will sell, no doubt about it)...