Avalon,
I would definitely recommend buying your own equipment if you ever plan on snorkeling again. We bought cheap sets for about $25-30, which is about what you pay to rent them at Castaway Cay. You still have to get a vest, but that's free of charge. In addition, you can buy a mask that fits you. You may or may not get a rental that does the job. The lines are not terrible, there's quite a few of them and they move pretty quickly.
We usually do some snorkeling on our own at one of the other ports, in addition to Castaway Cay.
ChiTownZee,
I'm not sure, but I think that the fins they rent are closed heel, and I would doubt that the snorkels have purge valves. It's pretty basic stuff. Keep in mind that I've never rented it, but that's my recollection from standing in line waiting for the vests.