Who uses them and what kind? I've never had one.
I use one to get a dobro sound from a pedal steel.
set the fader to a "comb pattern". not sure how it sounds on guitar never tried it.
(http://i26.photobucket.com/albums/c130/E9th/pedalboardsteel-1.jpg)
I have the DOD FX40B and used it as a solo boost. Worked well with one amp, not with the new one since I pretty much run it full on all the time. I have a Furman PQ-3 that I recently found after years of it sitting in a closet and it sounds fucking amazing in front of my amp. I keep it on all the time and just sculpt/boost the signal going into the amp input so that the right frequencies are distorted and the ones that don't do jack shit but muddy up the band (< ~75 Hz) are taken out. I think they're a great tool for sounding good in a band situation if you use them intelligently.
I have an EQ-Boost that I designed. Its great for making a small amp sound huge.
I have the Boss one which I'm not currently using and the 10 band MXR which I only have the 31.25 fader all the way down on to cut out extreme lows for guitar. My assumption is that that first fader controls frequencies between 31.25 and 62.5(the next fader) but if I'm wrong someone please educate a honky.