It definitely shapes the response, feel, and tone of the amp. It's not mind-blowing, but it's useful to dial in how wooly or sharp it responds. I've got an HT-5 for sale if you would like to give it a shot.
It's a useless gimmick. I had a TVP combo with this feature. For the first third of it's travel expect harsh, ear piercing treble, for the last third expect pure mud. Then try and find somewhere in the middle third where it doesn't annoy you too much.