I have used them and hate them. Most boats with curlers have in mast. There's more than one reason, mostly because they fail less and are much cheaper. They are not production boat type equipment they are super yacht rich buggers stuff.
But that's an argument for another thread. I'm not into that one.
FWIW, I have had both and have used both offshore. Pluses and minuses to both. I have greater fear with the in-mast for the usual reason, which is what to do if it jams halfway out, but the reality is that while you get better sail shape, full roach, etc., with furling booms (and that ain't nothin' by the way), they are indeed more finicky, and halyards jam too with a sail halfway up/down! The in-mast is very easy to use, much easier than in-boom. Frankly, from a convenience standpoint, the in-boom is only marginally more convenient, if at all, than a well setup set of lazy jacks or a Dutchman system.
Interestingly, the only trouble I ever have had with either system was with the furling boom. Gooseneck broke halfway to Bermuda, so we couldn't furl or deploy the main. It broke when the sail was reefed, i.e., halfway out, so we had a problem for a short while, until we were able to drop it to the deck, which actually took a little doing. In contrast, after 5 seasons, our furling mast has never given us a lick of trouble, whether offshore, coastal or inshore. But truth be told, I do still "think" ("worry" probably is too strong) about that potential jam with the sail half out, even though it never has happened to us.