I have the stompbox version - very pleased (got it on ebay). The effect is fairly subtle but is much more noticeable on clean settings where it improves clarity and 'lifts' the sound. I find on some heavy distorted amp settings I prefer to switch it off.
That's funny, I find the exact opposite :-)
I have a rackmount unit - the 482 - bought it after trying one through Feline's JCM800.
The effect on high gain sounds is amazing. Tons more definition & clarity. It's not particularly subtle either - the difference is huge. It did more to my Rectifier (which had quite a sludgy sound) than my Engl Savage but it's still very noticeable.
I find it does less to clean sounds - I like my clean sound warm, loose & swimming in Chorus/Reverb/Delay :)
Perhaps the difference is due to me using mine in the FX loop but you have it in front of the amp?