Went there in March, going again in Nov. It's decent diving out there on a liveaboard. I personally had a better time in the Similans than in Maldives.
I thought the Similan Islands themselves were decent, worth seeing. It was the other islands like Koh Bon and Richelieu Rock that were amazing. Koh Bon is in decline, as you can see large patches of dead coral where it must once have been incredibly beautiful. It's still better than the Similans, but must be far from its former glory. Richelieu Rock is still amazing. Have yet to visit Hin Daeng or Hin Muang, but will get that crossed off the bucket list in Nov.
There are a lot of boats, but everyone tries to spread out so you usually get no more than 1-2 boats in the same area at once. On the Similans themselves, we often were the only boat at that dive site, since there are so many to choose from. Others like Koh Bon and Richelieu Rock can get a bit crowded. Good buoyancy and safety sausage is a must for your own safety.
I've dived with the DiveRace E, had a good experience with them and will dive with them again shortly. They are towards the upper end of the price range though. You may also want to look into the Pawara as well as they have supposedly the greatest value for money. Book early as many boats, especially the Pawara, fill up incredibly quickly. I would avoid the South Siam unless you really want to save money (or it is the only thing that fits your schedule) as that is really a cattle boat.
Other scubaboard regulars like StevenL would have a far better understanding of the operations there.
I haven't tried the day trips from Phuket to Phi Phi or others yet. Liveaboards are so much more relaxing.
For Koh Bon you'd have to stay away from the bay to have nice diving with still nice corals.
