I recently replaced my furnace with a high efficiency gas unit. I opted for the humidifier, and leave the furnace fan on full time to help keep some consistency of temp and humidity on all levels. So far it is working out very well and I don't mind leaving the acoustic on the main floor on a stand. I also have the Oasis guitar humidifier, and find it does a great job in the case as well.
SVL, you mentioned mineral water... did you mean distilled? I imagine if you have fairly hard water, it could hamper the evaporation through the walls of the unit. So far I've only used tap, and no issues so far (8 months)
Gearlist:
Electric: Ibanez 'AS103', Fender Dlx Nash Pwr Tele, Fender Squier '62 JV Strat, Squier '51, Squier 60's Classic Vibe Strat, Epi Elite LP Studio, Hagstrom Swede Acoustic: Larrivee LV-03RE, A&L AMI, Yamaha FG340-T Bass: Yamaha BB 450 Amps: Roland JC-120, JC-50, Peavey Classic 30, Fender Super Champ XD Pedals: Marshall Guv'nor Plus, Danelectro Cool Cat Drive, Transparent Overdrive, Digitech Digiverb, Bad Monkey, Ibanez TS-9, Boss AC-2, CE-5, CS-2, DD-3, DF-2, DS-1, FV-100, GE-7, OC-2, PSM-5, SD-1, TU-2, DVM~BYOC 'Lush Puppy' Chorus