I have a smith machine, but I can't get my hands on the bar for squats, I'm too wide for the machine. I can do them, but it's a bit of a trick to unlock the bar from the rack and then not being able to have my hands on the bar during the lift is a bit odd, but in a weird way it does help with my form, really have to keep the back straight and not lean too far forward. Can't put too much weight on, but that's ok, good form with lower weights is better than a ton of weight and a trip to the ER.
Never been a fan of going beyond parallel, would argue the benefit wouldn't outweigh the risk, but if it works for others go for it. I'd just never recommend anyone to do it. Similarly never much of a fan of the box squat, too much pressure on the spine with no real benefit. If you do squats properly the box is not needed anyway.
Oh, this Twinkie thing, it ain't over yet. -Tallahassee