We dive regularly with Lahaina Divers when we're in Maui, and really like them. Mike Severn's and Ed Robinson's are both excellent operators too, and offer a different type of experience than Lahaina Divers.
Lahaina Divers has 2 very large boats (48') that typically take about 22 divers on-board. Even with full boats, there is plenty of room to set up your stuff and move around, and the dive deck is very large with 2 ladders. We've found staff to be excellent and very helpful, no complaints at all.
Mike Severns' and Ed Robinson use much smaller boats with only 6-12 divers. They claim to cater to the more experienced dive, though we haven't necessarily found that, and their dive sites are always interesting. Again, excellent staff and service, no complaints from me.
We dive with Lahaina Divers for a couple of reasons though. We stay in Kaanapali and Lahaina is FAR more convenient than driving for 30 min (or more) to Kihei. I'm prone to motion sickness, so I find the bigger boats to be more comfortable for me and I have fewer problems with them.
Another difference is that Lahaina Divers runs a regular schedule, so the divers don't have any input into where they might go. Of course one advantage to that is that you can plan your vacation around their schedule and know you'll do the dives you want. On the other hand, Mike Severns and Ed Robinsons will listen to diver requests, though the captain has the ultimate say in where the choice is finally made. Ed Robinson's also offers a 3-tank advanced dive, which I hear is fabulous. We haven't done it yet, but definitely will in the future.