I had an EXG a long, long time ago. It boosts the bass and treble and scoops the mids. I can't really recall what it sounded like, but I remember I preferred it for clean, low-volume sounds - I think it made the guitar sound a bit like an acoustic (probably not what it was intended for).
I've got a Clapton mid-boost circuit somewhere, but haven't used it because I'm a bit reluctant to rout my Strat for the battery compartment. :roll: