I've played a few co-op missions as well as a dozen pvp matches and the game is an absolute........the bottom line is that so far it's just fucking awesome.
The action in co-op is tense, it is a lot more difficult than many videos showed it before and the pvp feels very ballanced to me. I go with Titan class and its special/super abilities are pretty fun, once it's charged, you're a moving bomb, and if timed well it can be pretty devastating. (My record is killing 4 with one.)
I like the flood zone pvp map a bit better because the action is a bit more focused and matches are more intense, but the other map is great too, however that tank-like thing is a bit OP imo, as well as turrets. (It is possible that it's not OP, and it was only our team that was too incompetent to take it out.)
Weapons are great, especially the way you can unlock them (basically you just need to buy them).
Graphics-wise, the game delivers too. IQ is fine, textures are nice as well as geometry variety. Polished all-round, however the frame rate feels a bit chugy sometimes and I've noticed some tearing, but nothing serious.
I'm very pleased and I cannot wait to play the shit out of the retail version.
Thank you Bungie for giving us the opportunity to try it out.
Addition: I've been playing on a 200hz TV so the frame rate might've been a bit better for me (the perception). Also, the UI is the slickest and most beautiful I've seen in a game so far.