Sipadan diving is ranked among the best in the world and we decided that we need to see it for ourselves. The number of divers are restricted per day and you need to book long in advance your trip to guarantee you a day (or two if you get lucky) of Sipadan diving.
Most diving resorts are all inclusives and require a minimum of stays to be able to book. We did our trip with Scuba junkie and enjoyed it - all was handled very professionally. Also if you are not diving at Sipadan, then most dive sites are within 10min on boat from Mabul, which means your surface intervals are done at your bungalow chilling in your hammock !
The diving in Mabul and Sipadan is very easy and with great conditions, it's definitely suitable for all levels and especially if you plan on going through a course. I would also highly recommend the night dive from their pier - there is a lot of weird color nudibranches that you can only see at night !
Since all the organizing was done for you - it's very easy to get on this trip. It just needs to be planned ahead to secure a day at the national park.
On the way back we did plan for a super small time between the flights - we were stressed out ! I would not recommend doing this - even though we caught our flight.
Don't forget that the corridors in KLIA 2 are super long - it can take as long as 20min walk if you accrued all bad luck: The further away gate on the satellite and back if your next flight is also super far ! And plan for the usual 20min delay on all Airasia flights.
Also the capsule hotel is really just if you don't want to spend and in my honest opinion it's not worth it. There is ZERO noise isolation. Just pay the extra bucks and stay at Tunehotel or the aerotel that is now open attached to the premium plaza lounge.