I'm back from my trip, which I thoroughly enjoyed. I can now personally confirm that the info from the other posters is correct (not that I didn't believe them, but now I've seen it myself).
I'll just add one point on the non-diving side of things: There are two places to buy supplies. Silversands, inside the park and Mbazwana Spar in the town a few kilometres from the park/ beach. The former is open later on weekends. If you're staying outside the park, you're going to want to remember to go there straight after your dives or tell the guys at the gate that you'll be coming back, or you're going to have to pay to get in again later.
The vis wasn't at it's best, but I knew that December wasn't supposed to be the best month for vis (February is supposed to be much better, I think May is supposed to be even more so). And since I'm used to Cape Town's often rather murky water, it still seemed quite spectacular to me. The marine life was interesting: quite a mix of the more robust-looking, dull-coloured fish that you'd expect in colder waters as well as plenty of the brightly-coloured tropical reef fish. I've only ever seen one or the other at a dive site before. I presume it's because Sodwana Bay is so far south and the water temperatures are lower than tropical locations.
I was unfortunately 'that guy' -- I used someone's personal tank by mistake on one dive. I was a little irritated with the dive op, since I asked twice whether I could take any one of the cylinders from the pile, and missed the dot of nail varnish that someone had added to the valve to mark it as their own. I suggest marking things a little more clearly than that (unless you're not fussed with a mixup) -- the cylinders with people's names on them were nice and obvious and impossible to take by mistake.