Short recap: Sipadan is not a hype, it is absolutely stunning! Need to be a bit more lucky at Mabul / Kapalai but pretty okay. As a solo traveler I loved the resort, very easy to make contact with other people since there are loads ;)... Actually liked the village surrounding the island... Ow and bring Marlboro if you smoke them, didn't find any on the island...
To start with a bit of context: I did 65 dives at multiple locations in Asia and the Caribbean before I came here. Never dove with SJ before. I booked the Sipadan / Sangalaki combi pack and stayed on Mabul for 4 nights.
Trip: I came from Scuba Junkie Sangalaki and therefor was forced to stay a night in Semporna. If you have the chance to avoid this make sure you do so.... Disgusting town, it smells, hard to find a beer, overcrowded. Just no. Trip to Mabul by boat was very good organised.
Resort: I came from SJ Sangalaki which is really remote and relaxing so I was a bit shocked when I first arrived. SJ Mabul is like the complete opposite. Very crowded, big resort, loads of divers and loads of staff. After I got used to it I really liked it though... They run a very, very well organised operation. From the checkin to the food to the dinner, everything is just spot on. Impressive. Food was also really good, and more than enough.
Diving: You have to be a bit lucky with the equipment. Some is brand new but others are really (too) old. Once you find a good set, let the equipment guys assign it too you. The diving at Mabul and Kapalai is quite good actually. The diving at Sipadan is honestly world class and the best I ever had... Conditions can be challenging but all worth it.
What they do extremely well is planning the groups based on experience. At all my dives (in a group of max 4 +1) we came to the 70 bar point only minutes away from each other. Really good!
Dive guides: For me the only downside. Sipadan guides were really good and experienced. Some of the locals I dove with at the other islands were just not good and safe. Diving into NDL of < 5 minutes, constantly being the cool guy to other dive guides, banging melodiess on there tanks to other dive guides, that kind of things... Mainly just annoyed me but diving into NDL of <5 min I consider to be dangerous as well... Bring your own Computer.
Conclusion: I'll probably be back because of Sipadan! If you have the time it is worth doing a small tour while at the island and visit the other dive shops. Apparently they have some spare Sipadan permits from time to time...