I've got one, I've also got a set of Stauns. I've also bought them as gifts for a couple of other people. Never had any issues, or heard of any issues.
The Ferret works exactly as advertised - undo the valve stem.. about 1lbs / sec deflate, screw the stem back in, and Bob's your uncle.
Pros : They're fast. Way way faster than the Stauns. Can't compare with any other.
Cons : 1. Some people don't like the fact that you're fiddling with the valve stem all the time, claiming you could damage it somewhere at the back of beyond and then you're stuck. Might be a valid excuse, might not be, in almost 5 years of using one I haven't had an issue yet.
2. It's fast.. if you're with a group, you're going to be waiting for the others anyway.
3. Your back doesn't get a rest - bend over, unscrew, count, screw, stand-up.. next tyre... at least with the Stauns you've got time to crack a coldie.