Short, non-spoiler version:
Go and watch it for the CGI world of Pandora and the Na’vi. An amazing realization of an alien environment, the best so far. Ignore as much as possible the ham-handed plot that lacks all subtly and shows that Cameron does not understand what science fiction is. My Grade: B- (and only because the CGI is so spectacular, apart from that it would drop a full letter)
For my spoiler-laded analysis of why
it fails at being science fiction here.