Only partying is on Trawangan, the other two (Meno and Air) are much quieter.
Definitely worth going. Diving is pretty much all off the local outrigger style boats (though they are plenty big enough), nothing excessively deep. Current varies depending on location and tide but generally mild/medium drift to nothing, certainly nothing too scary. Viz around now should be 30m+ but season is a bit late, still ok at around 15-20m.
What is there to see? The odd shark but few and far between, sea snakes, loads and loads of turtles plus usual range of reef fish around these parts. Coral coverage varies from very nice to stretches of bits bombed to bits, overall though it is pretty good, especially the deeper sites where it is pointless bombing (this doesn't seem to happen anymore but obviously needs time to recover)
Most sites are close to island so out and back one dive, 2 max.
Been twice and stayed at Manta dive on Gili Air (got back from last trip there on Saturday hence up to date viz info). Would recommend them very highly. If you want to stay on Trawangan there is a Manta dive there as well.