I've been looking at starting to process C-41 at home; local processing is inconvenient and getting expensive for 120, and I really prefer medium format over 35 mm -- and there are some things that just don't work in black and white.
I see that Freestyle has their Arista C-41 chemistry available as powder (pint or half-gallon sizes) or liquid (quarts and gallons). The powder is quite a bit cheaper, and will be cheaper to ship (because no shipping water, which is heavy), but I won't have the option to mix a partial kit to extend the life of the chemicals, which means buying smaller kits and giving up some of the saving.
If I mix the stuff a pint at a time, I can do 4+ rolls in that pint (depending how much I'm willing to extend dev time and how many reuses won't visibly affect negative quality), which makes it cheaper than local 120 processing -- as long as I buy the gallon, and only unless the concentrates go off before I use them up. I don't shoot *that* much color; a gallon kit would probably take me six months or more to use up the capacity (and I'd be surprised if the opened concentrates would last that long). Quart kits are much less economical, but I could probably use a quart kit fast enough to go the powder route (that's only 8 rolls official capacity), and the powder is less than half the price of a liquid quart kit (so comparable economics to the gallon liquid kit). So, the question is how long would the concentrates last after opening?
Same question would apply to E-6, I suppose, though on that one I'd guess the color dev has a similar life to C-41 developer, while the first dev would be similar to D-76 for storage life -- and the E-6 doesn't come as a powder kit, but does come in a pint size...
(FWIW, I think the Arista is rebranded Tetanal; their instructions, for both C-41 and E-6, include times for temperatures as low as 70 F, which I've only heard of previously from Tetenal).