After recently diving the Galapagos, it was a toss-up whether or not to go to Sipadan. I'm skeptical about most dive locations no matter how highly they're ranked. All operators talk up how good their sites are no matter what. Sipadan was close by to where I was living at the time (Myanmar), so that ultimately made the choice for me.
If you like the backpacker scene, there's a pretty good set up at Scuba Junkies, based on Mabul. I had a few nights in cheap share dorms, and a few nights to myself in a deluxe room with aircon and hammock on the front porch which was perfect after a day of diving. There's also a beach out the front of the resort, unlike many of the other resorts that are built amongst the village or on stilts above the water.
To my surprise, divers only get allocated one day diving at Sipadan island, so most of the dives are around Mabul and Kapalai. It's mostly macro diving, and the conditions make it great for macro photography. Other than macro diving, I was lucky to see about 70 devil rays, dolphins during our surface interval, and an soldier armed with a M4 assault rifle who was stationed on Kapalai island protecting us from pirates following recent kidnappings. I made friends with the soldier and traded stories about the time I went to Nam.
Scuba Junkies have a turtle sanctuary where they collect eggs. I watched 70 green turtle eggs hatch in the nursery, adopted one (named Coleen after my nana) and released it. Only one in one thousand survive to maturity. Good luck Coleen.
In the end I managed to get two days diving Sipadan after another diver pulled out sick. If you like shooting turtles then Sipadan is the place to go. It has great conditions. Light and visibility are excellent, and no surge on the reef. Up to 70 metres viz in some spots. Loads of turtles! The other main attractions are large school of jacks and trevally, but to me this is not very exciting. I did a couple of dives where I took no photos! Despite this, I had an awesome time. For me, the place is perfect for a fun dive holiday, great relaxing on the beach after diving, but not the best place to go for adventure divers or wide angle photographers.