They work David, and Brett Dimichelle has played around with one. Where they appear to differ from say a tube amp (and I'm speaking in generalities) is that most tube amps have minimimal or no feedback. This tends to increase the output impedance of the unit such that it affects the FR of the amp as a function of the impedance curve of your speakers. Make sense? This unit, or ones similar to it, sit between the amp and preamp and you basically don't have this FR shaping altough you do get the 'warmth' or whatever word you want to apply here.
If you're considering investingating it David, I'd recommend that you head on down to a pro shop like Guitar Center (they're all over the US) as they discount heavily (bargain my good man!), have a fair money back return policy, can provide some pros and cons on the various ones from Behringer, maybe can turn you on to other manufacturers.
