I am a Scuba instructor and I own a Nikonos V, a Sea&Sea MX-10 and an Ikelite housing for my Pentax PZ-1 SLR. I wanted to move to digital photography for my Underwater Photography classes. (I am NOT an expert. I am NOT a professional photographer. I teach the intro course.) I also needed to reduce the weight of the gear I was hauling to the Caribbean. The Canon S30 with the DC300 housing lets me do just that. I added an Ikelite Substrobe DS50. (If you want to use an external strobe, you REALLY need to get one that is specially made for digital. Normal strobes don't work!)
I taught a class of high school students to do Underwater Photography last spring with this rig. I used it myself last summer on my own vacation trip. I'm using it again this summer in preference to my other rigs. We were all able to take remarkably good pictures and see the results using my laptop during lunch. I really liked being able to give feedback on the student's technique while s/he still remembered taking the picture. No waiting for developing.
(We also were able to take remarkably bad pictures as well. The delay between pushing the shutter release and actual snappage of the picture is Especially Annoying!)
As with any underwater photography gear, meticulous care and cleaning is necessary. That said, I found this particular rig very easy to maintain and very easy for my students to learn how to maintain and use.