I've never used one, but the concensus I've heard is that the slope of the shoulders of a carboy is not steep enough to effectively get the yeast to drop.
I'd worry a bit too about a high krausen reaching the vent tube and sealing the whole thing up. That would make one biiig mess...
I have never used one either. What I've heard, though, is consistent with what Danny said. Also, trying to install anything like a ball valve underneath seems doomed to failure as there isn't really enough room underneath to work it.
Not to mention, that clamp/bung/tube arrangement doesn't look the most secure. If something came loose or even started leaking, you would be without beer pretty quickly. But the ants would love all that sticky wort.
