I've used a few. If I were putting together a varmit/tactical type rifle they would be at the top of my list as long as you stick with the Medalist version with the aluminum bedding block. I've owned HS-Precision before and the B&C Medalist is every bit as good for about $100 less.
I wouldn't want one on a hunting rifle though. The versions with the bedding blocks are quite heavy and the metal inside makes for a thicker, chunkier stock than I want to carry around.
The Carbelite or Ti stocks do not have the bedding blocks and are no where near the quality. Because of the construction methods used they REALLY need the metal for strength. The better quality stocks made by High Tech-Specialties, McMillan, MPI, Manners, and Brown Precision use laminated layers of fiberglass or Kevlar cloth to build up the stock. This gives them strength without weight or bulk.
Most of them will drop in and will be free floated. It wouldn't be unusual to have to do some minor trimming in a few places. I've had to remove a tiny bit of material in order for the safety to move properly in the past, but it isn't that hard.
Short version. If you don't mind the weight or chunky feel, they are a good stock. I wouldn't expect any increase in accuracy, but it will be more consistent than wood and far more weather resistant.