I've had standard, in-mast and now have a behind the mast furler. It's a piece of cr*p and I can't wait to get rid of it. It's a Pro-Furl. It and other after market main furlers are no long offered which is telling in itself. The only good thing about it is that they can be removed and all the holes filled and restored although I think i'll just get a new mast.
Jeff pretty much got it right.
If you don't own this boat then use this point as leverage to get a better price.
Getting the sail out is a PITA. The boat has to be exactly into the wind and still it jams. The sail already has two patches from jam/chafe. (It does furl up pretty easily though.)
I hear everybody say how easy it is to reef and the safety factor. Sure, if everything is working but I know from experience things break at the worst possible times. If I can't get up to reef a main I can always cut the halyard and she'll come down. What can you do with a jammed main furler?
As for the in-mast, I agree with the convenience but I'd never buy another one.