Some of the physics in it seemed a bit dodgy to say the least, but as it wasn't central to the storyline it didn't really matter and it was more than made up for by the psychology of Sam's reactions to what was going on around him.
I was also impressed with Sam Rockwell, he's one of those people you tend see typecast lighthearted comic characters, so it was quite interesting to see him play a more serious character as it turns how he really can act.
I was also impressed with GERTY. Making him so like HAL from 2001, meant you were never quite sure whether to trust him and were half expecting him to go psychopathic and came out at the end feeling really guilty for thinking that as he really was sweet and friendly after all.
Having not really seen many trailers or anything about the film first, I wasn't really aware what it was about, other than an astronaught working alone on the moon coming to the end of his contract and about to head home when something happens (that said from what I can see they aren't giving much away in the publicity which is good), so when the film started and you saw GERTY, I had this feeling it was just going to turn into another 2001 style thing. However, I was impressed a soon as they introduced the clone twist early on. Especially the way they did it to make you think that the new clone was the old one and even when you began to suspect something you were never quite sure until you actually saw the body of the old clone in the space vehicle thing.
All of a sudden it changes from being apparently formulaic sci-fi to being a well thought out psychological drama as you see the two clones struggle to come to terms with what they are and the older clone dealing with the fact he was clearly dying.
All in all, it was an excellent film and highly recommended.