I used to have one, and it really depends on the amplifier. basically...
Amazing with: Marshall jcm800 or any old solid state
Bit rubbish with: Any other tube amp
Basically it has very little volume boost: the jcm 800 sounds exactly like it so it kinda builds on top i suppose, and solid state amps always take volume boosts well (jonny greenwood uses a fender solid state with it to get his sound). Thom has one but he mainly uses turbo rats I think. Any other tube amps just soak up the volume boost so it sounds a bit...stale I guess.
With a jcm800 its the balls though.