Dove with Scuba Junkies last summer.
The dive masters were good 1:4 ratio. It was a really hectic shop with lots of traffic & the gear was pretty sketchy. But, the prices were great.
It was hard to get a Sipidan permit then, now, given the new permiting system I don't know. Just make sure to make reservations with SJ ahead of time. SJ has a shuttle van that can pick you up from the airport.
If you're a budget backpacker, you can slum it in Semporna in the 12 person dorms above the loud air compressor that runs till late for $3 per night including a very basic breakfast. Private rooms w/bath were $18 last year. The bar is fun at night! Lots of young people. I think 3 dives a day cost US$90 & include a very basic lunch.
Alternatively SJ has an island homestay in a traditional village on Mabul Island. I think basic rooms w/showers (not sure if there is hot water?) are $18 per night. Scuba Junkies picks you up in the morning, saving you the long boat ride from semporna. I'd opt for this if I went back.
When I was there, the staff was overworked but well trained.
If you have your own gear & partner you should be ok.
If you use their gear & collect an insta-buddy - I'd suggest that you go early for gear fitting & select newer gear, test everything 4X's & stay really close to your partner & the pack!
If you have more questions feel free to PM me
messed up the viz (which wasn't brilliant to start with) as they were kicking up sand like there was no tomorrow 
