I think it functions best as a light gain device - it's a bit harsh with the overdrive too high. Basically I just leave it on all the time for a lo-fi indie kind of tone. The pedal responds wonderfully to the guitar volume - it'll clean right up as you roll the volume control down. It also responds great to playing dynamics - dig into the strings a little harder and it'll grit up.
Personally I think this pedal kills the Boss Overdrive and Ibanez Tubescreamer. It far more tonally transparent - those other pedals boost the mids significantly and shave off a lot of bass from the signal, trying to make you guitar fit their image of what an overdrive should sound like. Personally I prefer the grainy nature of the Hot Tubes drive - most other overdrives I've tried are too smooth, generic, beer-commercial sounding. I used to have an old 70s Big Muff that sounded better with the tone circuit switched out, but this Hot Tubes sounds a bit dull with the guitar volume low unless I switch the tone circuit in and brighten it up a bit.
Nothing is perfect and minus a point because it's not exactly versatile, at least not with my setup.
Hard to say. The unit I have has recently be reconditioned by Howard Davis, chief engineer at EHX in the 70s, but these pedals are approaching 40 years old now. I'd certainly have a backup ready just in case.
I like brighter, grainy overdrives and this is certainly one of those. In typical EHX fashion they went overboard with the size, but I can live with that. I've been through a few overdrives recently - the Boss OD-1, SD-1, Ibanez TS-9, MI Audio BlueBoy and this is the first I've really liked. The others were too boxy, unnatural and generic sounding. The Hot Tubes forte is light-mid indie rock overdrive. If you want to play heavy you'll want something else.