I was in Sipadan in February 2003. That is about as bad a month as you can get but viz was still 10-15 meters (usually at the lower end) but one dive it did make it to about 20 meters. Almost all dives are drift dives but beginners should not have a problem. In our group of 8, we had 4 with less than 20 dives each and they did not seem stressed at all. The marine life is fantastic and on several dives, I was getting dizzy from spinning around to see everything. For example, on one 50 minute dive at Barracuda Point, we saw 6 white tips, 2 black tips, 4 grey reef, 1 leopard (zebra) shark, too many turtles to count, 2 tornadoes of jacks (trevalies), 3 frog fish, a ghost pipe fish, a ribbon eel, some cool flounders, a few morays and cuttlefish, many bumphead parrots and the usual other suspects in their zillions.
I wore a 2mm shorty and was fine in 28c water. We stayed with Borneo Divers and I highly recommend them. Brian was our DM and was the best I have ever seen. I can't imagine what Sipadan used to be like but it is the best diving I have ever had.
In case you are wondering, I am a PADI DM with 300 dives in places like Lembeh, Minadao, Maldives, Banda, Grand Cayman, Phuket, Bali, Tioman, Aur, Boracay, Utila and Anambas so I have somewhat of a decent perspective (although I know there are 1000's of places I haven't been).