Yeah you theoretically wouldn't have to rack to secondary and could easily collect yeast for re-use too.
I've never used one because they look scary to me. I really hate breaking carboys full of beer.
I have heard from people who use them that the shoulder of a carboy is not steeply sloped enough and a lot of yeast/trub ends up perched on the shoulder of the carboy and you can't get it out. Still, I imagine it helps.
I don't know. I have thought about it, but it seemed kind of a hassle for little gain. Someone on here must have one, though. Cheers!