Did a long weekend in salt cay last summer and it was great. There is literally nothing to do except hang out at the restaurant by the dive shop or porter's island thyme (which is being sold). Stayed at castaways, owned by porter (of aforementioned porter's island thyme) and Haidee, although they're selling the restaurant which I presume may mean they're selling the hotel as well. If you are truly liking for solitude look no further.
The dive operation is top-notch. Debbie owns the dive shop and Richard is the DM. There were 6 of us and Richard--he just anchors the boat and gets in the water with you. (We were there right before they close for hurricane season so this may change in the high season.). The main bonus of diving there is it is a 5 minute boat ride to the diving. You return to the restaurant for your surface interval. This makes for a lot of R&R. It's not for everyone, though, since it's a sleepy little island.
That said, if I go back, I will spend a bit more time in Provo and/or possibly do the aggressor (assuming I enjoy the liveaboard diving I'm doing next week.
). But if you've got some time, want to get away from it all, and don't do a liveaboard (or have some extra days) it's definitely worth a few days on the island.
One thing to note: if you go to grand Turk you may try to time it so you're not there when the cruise ships are docking. Don't know if that's possible, but I understand there are scuba excursions that can be annoying.