Personally I don't care for them. I've seen a couple of boats at my marina take on a lot of water when something came loose or the bellows cracked. Also, I'm told that you need to be careful to "burp" the thing after a haulout in order to get water back into it. Without the water it can get awful warm in there.
Boat: CS36Merlin, "La Belle Aurore" Ben393 "Breathless"
I've got one in my Beneteau. Works very well but if I had my druthers I'd not have one. Prefer the traditional stuffing box. At least I'd know what to do when it starts leaking. Yes you do have to burp it after a haul-out, put a bit of grease in it too. So far so good.
I broke my drive saver 700 miles from any where last Fall. When the drive saver bolts sheared off, the shaft dropped down and the dripless seal suddenly became a pour-thru seal. Luckily, by realligning the shaft, the seal again became dripless until we could get the drive saver replaced when we got to Tortola. Would a traditional shaft log have let in as much water?