I think it's a pretty decent selection. Probably not enough that I'd sign up for Prime solely for it, but still decent if you're into older movies/shows, documentaries etc. Not on par with Netflix, but it's also not a pure movie subscription.
It's worth it for me since I order from Amazon almost weekly and like the fast shipping and not having to pass on deals for cheap items due to not wanting to pay shipping or buy more other crap I don't need/want to get up to $25 total.
And having a Kindle, I can read a book a month through the lending library for free with Prime. And I see several titles I want to read in there, so that saves some money vs. having to buy the ebooks and helps defray the cost some.
So I'm very pleased it's on PS3 now as it's nice to have some streaming options again since I canceled Netflix streaming when they started charging extra for it. I'm lucky to watch more than a handful of things on streaming a month, so it's not worth a pure subscription for me (I mainly watch my disc rentals and things I own, or things on TV currently like sports). But it's a nice option to have when bundled with the other features (fast shipping, kindle books) that are more useful to me.