I saw it last night, and while it was slow, it was also intense, and I enjoyed that intensity enough to give it a strong 3½ stars out of 5.
It was refreshing to see a sci-fi movie that didn't involve a raft of over-dazzling special effects, an inimical alien hell-bent on killing everyone in sight, and a super-hero dashing in to save the day.
NOTE: Someone in Hollywood needs a primer in orbital mechanics. I mean seriously, an unplanned stop off at 1685 Toro during a 19-day trip to Mars. Has Hollywood ever heard of ∆v?