In 2009 I'd have given it a 9 or 10. Now, maybe a 7. If Pandora at Disney becomes huge and builds buzz, I'd give it an 8 or 9. If the first sequel is good and on time? 9 to 10 again because people will keep coming back. Look, Cameron is a huge name. His major projects lists have pretty much been all hits. Several spawning franchises. Avatar is his baby. I just don't see him making a bad SciFi movie, and if it is good, people will flock to it. He is always at the cutting edge of special effects, and that is what you spend the big bucks to see in a movie theater as opposed to waiting to see it on the small screen at home. So I really don't see it as a huge gamble, other than the original has gone stale due to the ridiculous lag.
Here are the movies he has been both director and writer on according to Wikipedia
Xenogenesis 1978
Piranha II: The Spawning 1981
The Terminator 1984
Aliens 1986
The Abyss 1989
Terminator 2: Judgment Day 1991
True Lies 1994
Titanic 1997
Ghosts of the Abyss 2003 (Documentary)
Avatar 2009
Which of these movies that he wrote and directed, as he is doing with the Avatar sequels, has you questioning his ability to make a good sci fi movie with a strong following? I mean, I get questioning 1978 and 1981, but after that? These are all huge hits, except maybe the documentary. I have no idea if that was a hit for a documentary or not. The man makes good movies when he has control, and only good movies.