I have a Q-Tron+ and it sounds pretty sweet. To be fair though, they aren't just plug and play devices. You can spend a long time just messing around with the things, and they are very sensitive to the original signal - I find I have to max out the signal from the guitar into the box to get it sounding right.
Even when you have the sound right that's only the start of the tonal possibilities. I have a Boss OD-3 and I can't decide wether it sounds better before or after the Q-Tron on the pedal board.
But I can tell you that nothing beats hearing that glorious Dm chord at the start of Shakedown when you get everything dialled in