I will be going to Sandals Grande St. Lucian in April 2008 for my honeymoon. :) I have looked over tripadvisor.com, talked to friends, posted on forums, and my travel agent (who has been to all the resorts many, many times). It seemed the best choice for us. We like warm temperatures, calmer waters, all-inclusive, etc.
I have been to Beaches and Sandals in Negril Jamaica. I really like how this type of resort is run, so that's why I wanted to go to Sandals for the honeymoon. I did not want to go to a family/kid allowed resort. I have nothing against kids, I just didn't want that type of atmosphere on the honeymoon.
If you stay at one Sandals in St. Lucia, you can visit the other two for free. So, that gives you some "safe" options. I also would like to ride the horses on the beach/ocean if we can. I did this in Jamaica and enjoyed it a lot, but my fiance has never tried...so he wants to give it a shot. The tour desks in the resort can help you a lot with this. I'd also check out the tripadvisor.com forums. From what I've heard there are quite a lot of things to do, including Pigeon Island National Historic Park that you can walk to and hike up. Let's also not forget the twin volcanic peaks, the Pitons.
Also, they are having KILLER sales right now with Sandals. Grande St. Lucian has their prices at 40% off for April 2008.