I absolutely love this island. I have been to St. Lucia three times and have recommended the island to several of my friends (they all ranted and raved about this island as well).
Most of the resorts are located in the northern part of the island just north of their capital city, Castries. There are two airports that service the island; one in the south where the large jets land and a smaller one in Castries called Vigie airport. I strongly recommend you fly on a commuter jet from San Juan, Puerto Rico, to Vigie airport. It should only be a 10-15 minute drive from the airport to your hotel (instead of a 2+ hour ride on a windy, ill-repaired road from the southern airport).
I stayed at the Windjammer Landing resort the first time. This was an excellent hotel; however, it's a bit remote so you can't walk to anything. I stayed at one of the Sandals resorts my second stay back in 1998 or so. Sandals now has 3 resorts on the Island and I've known people who stayed, and enjoyed, all 3 resorts. Last time I was on the Island in 2000, I stayed at a place in the middle of everything near Rodney Bay. It was nice being in this section of town as restaurants and shops were in walking distance of my hotel.
The people there are very nice. However, it is a third world country (as most of the Carribean islands are), so be prepared to see poverty. With that said, I had no problems walking around and talking to the locals. However, be careful in what they try to offer you. One person tried to give me a package containing, I'm sure, an illegal substance. This is the carribean, after all.
For activities, I highly recommend you take the rain forest tour. What fun! I absolutely loved it (and, I believe I had my best meal while on this tour). The water on the Carribean side is beautiful and WARM. This is a volcanic island, so several of the beaches are fairly small. Snorkeling is great near the Pitons. I believe that the snorkeling on St. Lucia is ranked one of the top 10 (0r 20) in the world.
I also did the catamaran trip down to the southern part of the island where you go to the drive-through volcano, saw the beautiful botanical gardens, followed by snorkeling in Marigot Bay (they filmed portions of Dr. Doolittle here).
E-mail me if you have additional questions. I absolutely LOVE this island!
Lorrayne