I've heard they're not good for extreme heavy loads, due to the un-natural pressure it puts on the hips and lower torso. I've gone as heavy as I could before, and never felt any pain in those areas. On the flipside, there was a guy in my gym who refused to use one. Said the same thing about it being an un-natural movement and pressure. I generally use one only for low weight, high rep burn sets, with good ol' squats as the King of legs.
I suppose if I had the space for one, and it was a stupidly good deal, I might jump on it. If I couldn't or didn't use it as much, I could sell the machine, and maybe keep the plates, flip it for more money. Something like that...
Mt Snow, VT - May 5, 2012 - Completed!
...i remain, he who remains to be...