It's not super long, maybe a few hours depending on how much skillzzzz you got. The first half is amazing. Awesome puzzles, great looking, moody atmosphere, great (lack of) music, creepy and un-nerving.
The second half kicks up the puzzles, but they become a lot more reflex, timing based (ie. jump here then jump again before the spikes crush you but not so high you hit the spikes above). The game kind of wants you to die these gruesome deaths to learn "oh. I can't step on that huh?" but the reloads are instant so you don't really get frustrated. Replayability lies in finding hidden 'eggs' in the levels, some of which are crazy hard to find/get without a FAQ (and even with a FAQ some need perfect timing)
I really really liked it. A lot of people bitched about the $15 tag. It is a bit much dollar=time ratio but it's really well polished so I didn't mind paying an extra $5 over the $10 I-think-it-should-have-been price tag.