I just got back from St. Lucia on Sunday (8/8). I was there for a week and stayed at the Windjammer Resort. I had a blast. The resort was great, the water was warm, and the dives were great.
I dove with Dive Fair Helen in Castries who were recommended in one of the above posts. They had great service, a big enough boat, and picked some good dive sites. I did the 2 day/2 tank package and we dove at the base of the Pitons. The four dives I did were: Superman's Flight, Trou Diable, Gran Caille, and Pitons wall. The dives were great reef dives. The first one was a drift dive and we were really moving (I guess that's why they call it Superman's Flight). The others were much calmer and the reefs and fish were amazing.
I second the recommendation for Dive Fair Helen. They pick you up at your hotel, get you on the boat, give you lunch, and take you to the dive sites. The dive sites took us about an hour by boat to get to. We were picked up at 8:10 and were back at Windjammers around 4 each day.
I dove with all of my own gear, but the gear that other dives were using was rented and it seemed to be fine. It was all scubapro and mares and seemed to be in decent shape.
Let me know if you have any questions.