When I was recording on my 4 track I would use them on the final mix and it helped the mixdown tape sound better. Now that I have a full blown DAW I almost never use it.
My guitar player uses one live plugged into his amp and it definately makes a huge sound difference, much more depth and fullness to the sound. You just have to make sure your not overdoing it. If you use it on a guitar amp and it sounds like crap it may be a defective unit.
But I think they have more of a live aplication than for recording (although you could use it on a guitar your going to record).