We just returned, well back in early August, from the USVI and can highly recommend St. Croix. We'd been to St. Thomas, St. John, and the BVI before but never to St. Croix and OMG what a difference. The diving was excellent and there was a lot to do on shore.
We dived with St. Croix Ultimate Bluewater Adventures and they were wonderful. We dived from both of their boats so we were able to see many different sites. The wall there, especially Salt River was incredible, there were some great wrecks dives that we went to one day then we did the Frederiksted Pier and it was amazing. We went to Bonaire two years ago and I've got to say that their Pier was pretty good but I liked the one on St. Croix much better and we were the only ones diving it on a night dive we did there.
I think St. Croix is great for just about any level of diver. There's no really "challenging" dives, ie: strong currents, overhead environments, etc. With the walls there you can certainly go as deep as you'd like but you obviously don't have to.
As for other things to do, it's a pretty great place too. There's a great botanical gardens and it's near the Cruzan Rum factory (brewery? distillery? you can tell I was more into the rum). The snorkeling is good all around the island and there are some wonderful restaurants.
