Thanks for the replies, so far. I definitely understand the appeal of amp-sims to use at home/in the studio, I use them for that myself.

I should say, I'm more interested in pedals such as the Benson Preamp, Zvex box of rock, JHS colourbox (?), EQD Acapulco Gold, etc. AFAIK, theyre supposed to be used as gain pedals, not amp replacement/sim pedals.

So what I'm specifically interested in, is in which cases it's preferable to use one of these pedals, when volume is not an issue. When you have something like a Roland Jazz Chorus that you want to add some gain to? I imagine you wouldn't want to use them with a Bassman or Marshall, etc.