There are two issues that keep from tempting me - and that is the 'thankfully' part. 1) 100 degree temp requirement vs the closer to room temp 68 degrees for B&W. Sure I could over heat and ice down...but it is the holding it for 30+ minutes without expensive dedicated heaters. and 2) the relatively short shelf life - weeks vs months for B&W - for mixed chemicals. Even with something like Arista's quart solution @$35, it will process 4 rolls. Will I shoot enough (or save exposed rolls to accumulate for one developing session) to justify.
Even with the expanded shelf life of B&W, film is so niche with digital work dominant, I've enteraine the though of selling the 35mm cameras and lens, and purchasing medium format 120 camera. 12 exposures rather than 24 or 36 before developing and 120 processing uses the same quantity of chemicals as 2 rolls of 25mm. Of course, 120 slide film would have to be digitized as don't have a projector for that size.
Here's sort of the key with reversal film developing-the only true time-temperature step is the first developer. Each subsequent step is done to completion, so if the temperature is a bit cooler it's easy to just extend the time of the subsequent steps a bit.
When I did it, I filled the bathtub up with water. I kept EVERYTHING in it-my mixed chemical bottles, bottles of wash water, everything. Basically what I did was get it as hot as it would come out(which IIRC was around 115º actual measured) then let everything equilibrate in it while I watched the temperature. If it went below 104º(which I think is the specified processing temperature) without everything actually at temperature I'd drain some water and put more hot in. As it was getting close, I'd dump a pre-wash into the developing tank to help the film get warm.
Once it was all at a good 104ºF, I'd start the first developer and it would hold close enough for the 10 minutes or however long it ran. Usually by the time I got around to the final rinse everything would be down to 90º, but again the lower temperature is easily compensated with extended time and those are better to err on the side of longer.
Some people do turn their noses up at the Arista 3-bath kit as it's not a proper 4 bath or 5 bath, but only having 3 processing steps is a benefit for my DIY hot bath set-up.
Also, as to your comment on 35mm vs. 120 developing volumes-remember that 35mm-36 and a single roll of 120 have aproximately the same surface area(as do 4 sheets of 4x5). I seem to remember that the Arista kit wants you to use 16 oz. of chemistry per "equivalent" roll, so whether you're doing 35mm or 120 you'd still be using a 16 oz. tank and 16 oz of chemistry. Don't hold me to that as I only ever did sheet film and 120 at home, not 35mm. It's not unlike how if you dilute D76 1:1, you really should use 16 oz. to develop a single roll of 35mm(even though I've gotten away with 8 before, or with doing 2 rolls in 16 oz).
