I dove at Ko Lipe in Nov last year. Went with both Starfish and Forra--both good competent ops. I did this as a sort of a side trip on my way back to Singapore after a liveaboard to the Similans--so didn't go hardcore on the diving. I'm not sure when the tourist season really starts, but it seemed fairly quiet when I was there. Beautiful beach on the south side of the island with pretty good snorkelling...
I didn't have great viz when I was there--especially after the Similans. The local sites were okay, but there seemed to be an awful lot of fishing going on--ran across lots of traps. Ko Rawi and the surrounding islands are part of the Ko Tarutao National Park and are nominally protected--but it seemed a lot looser in practice. There seemed to be a kind of uneasy truce between the dive ops and the sea gypsies, but Ko Lipe probably doesn't generate the kind of diving $s that the Similan/Surin's do and I didn't really see any evidence of any protection.
8 Mile Rock was a great dive site, but I had horrible viz--it was like being back in the Northwest, ~10 ft max. I do remember having a phlanx of batfish follow us around, and I remember dropping down almost right on top of a leopard shark. The site probably would've rivaled the Similans if it hadn't been for the viz, but it was a full moon so it probably gets better. Dramatic pinnacle--with a big scar from an encounter with a passing fishing trawler. Kind of a long ride to the dive site by longtail though...
The dive shops were doing some exploratory dives when I was there. Prices seemed about average compared with the rest of Thailand. A nice getaway and a little off the beaten path--but maybe not the place for a hardcore scuba-intesive trip... Seemed mostly like lots of couples there for a romantic getaway. More europeans, less americans.