Hi Leafy, to answer your direct question first, yes, Sipadan/Mabul is worth it. Definitely. And you should know that it is Sipadan/Mabul in a very real sense. You will stay and dive in Mabul and the number of days you will go to Sipadan will depend on the quota system your resort has worked out. When you go to Sipadan, you will get 3 dives. I stayed on the LOB Celebes Explorer, as did Hintermann above, and they will get you to Sipadan about 5-6 days out of 7. By the way, Sipadan has only 7-8 sites they dive (even though they have 15-20 on their map).
But you are missing a big part of the picture by fixating on Sipadan, legendary though it is, and well deserved. Diving in Mabul is fantastic in it's own right. Excellent macro, and huge groupers, huge turtles, amazing night dives (the night dives rivalled RA in my opinion), seeing turtles swimming by at night was amazing (of course, no fun for them that we were waking them up probably).
And if you don't mind, I'll add my 2 bits worth regarding your overall itinerary. I don't know how much time you really have, and I know that many people like to go somewhere just for the experience of having been there, even if for just a day or two, and if that's what you want, great! My own opinion is that since I am very unlikely to go back to that same location again, I want to make sure I don't regret leaving it too early. If that makes sense to you, I'd suggest skipping Bunaken. The diving there is fine, but not unique, and the time should be added to Komodo. For sure. Or, if you are set on Bunaken, spend more time there and on Sulawesi (have you considered Gorontalo?) and maybe leave Komodo for another time.
Also, Sipadan is not far geographically from Indo, but the flight logistics are not easy. If you do go to Sipadan, Maratua is not far from Sipadan (very close actually) but again the logistics are not easy (neither flight nor ferry). The seascape and fish are very similar. I actually had much better experiences with barracuda tornadoes (and jacks) in Sipadan compared to Maratua, unlike Zippsy, but that's diving for you! You will need reef hooks in the Maratua channel. But from Maratua you can go to Kakaban lake and as Zippsy said, snorkelling with the non venomous jellyfish was fantastic.
Let me know if you have any specific questions. But the first question is to you: How much time do you have? Really have. The diving in all these places is fantastic. You'll have a good time, regardless.
