Just got back from our HM in St. Lucia! We stayed at the Regency for 6 days, and the Grande for 6 days. If you can stretch your HM out for more than the typical week, we would definitely recommend it- otherwise, to us, it just seemed like a normal vacation.
Everything was great. St. Lucia is pretty, but the roads are VERY windy- to the point where people were getting car-sick on the ride to the hotel- which to the Regency was a little over an hour drive. So, if you get car-sick easily, I would definitely recommend either A) taking the helicopter ride ($189 a person), 
taking a private taxi, or C) taking some type of motion sickness medicine beforehand. Also keep this in mind for any excursions you may book.
The Regency:
This hotel was great. We have no complaints. It's somewhat spread out, depending on the room you book. If you do book one of the higher rooms not in the main building, there is a van that runs every 5 minuets to the other area (where Armondo's restaurant is too). The food at every single restaurant was good. You do need reservations for La Toc - the French place, and Kimono's - the Japanese Hibachi one (this one was my favorite). The main pool is fun and always has activities going on- including volleyball and basketball. And, there are two other quieter pools - one by Piton's and one up by Armondo's as well. The beach is a little small, meaning they don't keep charis out on it because the waves come pretty far up. The golf course was fun, but it was sooooo hot! The spa was good as well - we got a couples massage. All the people are super nice who work there, and the activities at night (different theme every night) were cool and a lot of fun. They also have a piano bar which is where the entertainment is after the main entertainment end that is OK - it's somewhat small and very loud when they put a DJ in there.
The Grande:
Although I liked all the activities at the Regency better, hotel-wise, the Grande was my favorite. The food was awesome, but I think that is the case at all Sandal's. We had a concierge room here, and everyone was so nice and knew our names every time they saw us. The hotel itself seems a lot more personable to each couple. It has a few shops, and a bunch of restaurants. The beach is by far, much better at this resort as compared to the others. And, the water activities seem more fun/easier because the Grande is on an inlet as compared to the Regency which is on the ocean. The spa here was great as well. We splurged ($300) and got the "Scrub a dub dub - two in a tub". It was amazing- a couples massage, then a couples exfoliating scrub, then they leave you alone in this room and you hang out together in a hot tub, and then they moisturize both of you - AMAZING. They also have nightly entertainment here which is fun and changes each night. Although they have activities by the pool, they are not as fun/involved/many as the Regency.
The Halcyon:
We just visited here for dinner at The Pier Restaurant. The food was good. As far as the resort goes- it's a lot smaller, and a lot quieter. There is no large buildings, just one level rooms/cottage type things. They don't have entertainment or activities there that I am aware of.
Excursions:
DO NOOOTTTTTT book the volcano tour through the hotel. It was a 5 hour tour which for most of the time you are in a van riding around on the windy roads of St. Lucia. They stopped in some village on the way, basically so you can buy stuff. Then we went to the volcano in which (I kid you not) we spent 15 minuets there (couldn't go in the sulfur springs or anything). Then on the way home, they stopped at some "restaurant" so we could eat - although no one wants to eat because of the ride back. It was horrible.
If you do want to visit the volcano, I suggest you book one of the tours that are being offered by people on the beach at the resort. This is not through Sandal's, but we met many couples who signed up right there and there and had a lot of fun. I think the cost was $100pp. The tour takes you by speedboat to the volcano, and a van actually takes you in. They also let you go in the sulfur springs mud bath. They also taking you snorkeling, cliff-jumping, and to the waterfalls on the same trip. Much better than what we did.
The only other excursion we did was zip-lining, which we booked through the hotel. It was tons of fun, and the ride isn't that bad. There are 11 lines and they take you on all of them. I would definitely recommend this tour.
I will post pictures in the next few days so you can see!
If anyone has any questions, let me know!