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