I use Spotless for all my brews and have done for about 9 months now. I've got an RO system and to be honest I've not used it since I got a Spotless account.
You don't have to worry about the water being sat there in the tanks for ages....the prime users of the Spotless system are window cleaners who fill up their systems tanks with huge regularity. Its quite common to get to the Spotless outlet and find a queue of folks waiting to use it. My local spotless outlet is 25 miles away from where I live....I can drive there, fill up 40L ( takes about 45 seconds) and drive back in a quarter of the time than it takes for my RO system to generate 25L of RO water.
40L will cost £1.60.....it would take 25-30 brews before I end up spending more on my Spotless water than it would on a basic RO system....throw in the time taken to generate RO water from a domestic system and for me its a no-brainer.
I never, however, use Spotless water as my sole brewing liquor.....it is always cut with my local tap water (which is pretty hard)....maybe 85/15 (spotless/tap) if brewing pale beers, 50/50 when brewing stouts....basically, whatever ratio is needed to keep the actual physical salt additions needed, to a bare minimum (there is no point taking stuff away only to re-add it again)...plus there are other beneficial nutrients (e.g Zinc and Copper) within tap water that would not get reintroduced by the "normal salt additions" that one would make.....so in my opinion it is beneficial to always use a little bit of tap water.