Hate to be a party pooper but here's the list of players that had a PER of 7 or below in their rookie seasons:
http://bit.ly/Zz8uAH
Not inspiring at all, however, that is to be expected -- a PER of 7 for an entire season is bad - like historically bad (especially at the amount of minutes that Rivers is playing).
http://bit.ly/U8DSOM
But, that's a very exclusive list. If we extend the number PER to those less than 10 then it becomes less depressing. There are players who've had bad rookie seasons that eventually carved out a niche for themselves: Sefolosha (8.5) became a 3&D, Diaw (8.5) became a pass first, pass second big man (LOL but you get the point) that flourished in PHX, Brandon Rush (9) also became a 3&D, Vasquez looks to be a good backup PG.
Especially Rush -- who was one of the worst players in the league for a couple of seasons before GSW was able to find a good role for him (and before tearing his ACL).
So it's not hopeless. Also, his numbers are trending up
Defensively, he's inconsistent -- but so is the rest of the team.
He still has a long ways to go to fulfill his projections as a "star" but it's still not known at this point whether he's a bust. Too early to tell. Wait till the latter half of his 2nd year OR early 3rd year to REALLY judge whether he's a bust.