Just to add my 2 cents... I was asking about humidifiers last year when I was experiencing bad clogs on my 4900. I purchased this
Sears/Kenmore unit and it's worked great. I don't have a water source to continuously fill it, so I have to fill the tank every morning, and again when I leave at the end of the day (my studio is out of my home). I'm in Central Ontario - north of Toronto and it gets very dry here in the winter... 25ish % a lot of the time. I try to keep mine at least to the 40% mark but prefer it 45 to 50%. In that range I rarely have problems.
I just got back from not having the printers (I also have the 9900) or humidifier running for two weeks. Before I left, I put some AIS fluid in the docking station. I still had a clog on the 4900 on almost all channels when I got back, but I got the humidity back up to 45%, did a head clean and print - and the print was fine. It's still showing a few misses in the test pattern, but it's working and now that I'm back and can keep the humidity at a reasonably consistent level, I should have few problems.