i saw it last night. i was prepared to be disappointed, given the level of expectation. basically, if you go in with an open mind, not expecting a film just like Alien or Aliens, (and certainly not a typical summer blockbuster!) you shouldn't be disappointed.
i found my self lying awake thinking about the film, trying to piece things together, and wondering where it would go next. that alone makes it good to me. usually you walk out of the cinema and pretty much forget about the last two hours!
the 3D version is good, by the way. not gimmicky, thrusting into your face. it just has a wonderful sense of depth, like looking through a window into another world.
a lot of people say Scott's films seem cold, soulless. this won't change their minds. and they're still wrong :)