I did a trip on the MV Celebes Explorer in April 2013.
We did a checkout dive in Semporna area while waiting for everyone to arrive but were at Sipadan for the rest of the days. At the time, we were allowed to do 4 dives a day at Sipadan (there was some talk of being forced to cut back to 3) and then went over to the Mabul area to do a night dive and park for the night. There was a morning with extremely rough conditions and we did not make it over to Sipadan until the 2nd dive. So...we probably had 23 dives in Sipadan. The diving was spectacular (including night/mandarin dives in Mabul, which are among the best night dives I've ever done).
The downside is that the boat was in rough shape and the captain & dive guides were downright awful (cook/cleaners were always friendly). Luckily, we only had 7 divers for 12 spots, which meant that all of the singles could have a private room. I can't imagine sharing one of those tiny rooms with another guest for 7 nights. Very cramped with minimal air ventilation & the occasional diesel smell.
There were no "good morning's" or "how are you?'s" from the captain or dive guides. If you were lucky, you could get them to acknowledge that you were actually in front of them. I had a brand new mask and a nice bottle of Scotch disappear. Could the mask have fallen overboard? It's possible. I doubt it though.
Although priced at $300+/day, the boat is closer to what you would get in Thailand for $140/day (but at least the Thai boats hire a nice old lady who's been cooking for half a century!). Would I go back on the Celebes Explorer? Maybe. I'm actually thinking of another trip to dive Sipadan and can't decide if I should to something land based like Scuba Junkie instead. I'm terrified of having to share one of those tiny rooms on the Celebes Explorer with another person. There are no single rooms and the single supplement for a private room is somewhere around 90% of trip cost. The food was cafeteria quality and certainly lacked any creativity.
Almost 150 dives later, and in some pretty awesome places (Indonesia, Philippines, Maldives, Bonaire, Alaska, South Florida), I still find myself comparing dives to Sipadan. Amazing diving. Crappy, overpriced boat.