Do keep in mind, you will have to keep buying filter cartridges for your unit, as well as the RO membrane on a regular basis. If you neglect buying the prefilters, the RO membrane won't last as long either. These can run $5-20 per filter and $30-60 for the RO membrane.
I have wanted to get one for a while now, but I opted for connecting 2 more plastic barrels to my first one, and now with 2 or 3 together at 55 gallons each, I have enough water to last me.
I collect it in 5 gallon containers during the rainy season and store as much as I can in the garage. I now have enough left over to water the garden a bit too. All from one gutter over the garage! I wish I had more connected to the rest of the house!
It is a bit of work, but it keeps me in shape as much as possible, and with my lack of funds it saves a lot of money for me.
If you are humidifying a small grow chamber, you shouldn't need more than a gallon a day anyway.