I've been using a GE-7 in front of Marshalls for lead boost for 20 years. (2203 'way back when, now a homebrew 18-Watter knockoff.) I have not noticed hiss or noise, but I generally just switch it on at the start of a solo, and right off again at the end. I like the overdrive it gives by slamming the preamp 12AX7 way more than any distortion box I've ever tried. It also allows you to re-sculpt your EQ curve in boost mode. I generally run it with the sliders in a slightly lopsided, inverted U shape, with the upper mids boosted a bit more than the lower mids, little or no boost at all at the ends.
Other satisfied GE-7 users that have not been mentioned above: David Gilmore and Steve Stevens.