At Castaway Cay I used a disposable to try to get some shots of the underwater scenery (especially the nautilus, of 20,000 leagues under the sea ride fame). I was quite disappointed, basically it massively under exposed everything. I always used to wonder how they were using a fixed exposure camera to get a wide range of exposures. Now I know. They accept that it might underexpose and just let the processors try to fix it in post. The result being that I got murky, grainy images. Even though I was in shallow, clear water in full sunlight.
To me that's the payoff of a housing. I probably wouldn't use it much, but at least I'd get adequate pictures out of it. The disposable camera was reasonably inexpensive, the processing doubly so, but the results weren't at all satisfying.
I'll see if I can get some of them scanned.
I'm actually thinking the solution for me might be to get a waterproof point and shoot. The fact is that I can most likely get one of those for less than I'd pay for a housing for my DSLR. There aren't many to choose from, there's a particular Olympus model that seems to be about the only real choice except for a number of no name brand cheap setups, and that particular model has a reputation for having poor focusing among other things. But it's small, convenient, and sufficiently waterproof for snorkeling. And I'd get a camera that I could use as a normal point and shoot as well, plus on water rides or in the rain.