I use an Audio Desk machine and love it for the simplicity. I suppose I change the water every 70 LPs or so, mainly because I go slow these days and by that time it feels about right to make a change. A few points:
I agree that the machine appears to leave a residue and that you can get better sound by rinsing the LP afterward with pure water and a vacuum, or by using a VPI machine instead, for instance, with good fluid like the Audio Intelligent stuff I use. I don't like to admit it, but I do hear an improvement with the AI fluid over the Ultrasonic. But again, it's the simplicity that counts for me - if I had to do every LP on a vacuum machine I would never get around to cleaning my LPs. And having invested money in some decent carts...
You can clean off the brown crud that accumulates on the brushes after extended use. I do it by removing the two rollers and rubbing them together while dunking them in distilled water. Each one cleans the other. They look great after this. Might save you a round or two of purchasing new cleaning brushes.
on the frequency of use: According to the rep in PA you should clean records every three weeks at least, if you keep the machine filled with fluid. Otherwise the water can degrade the water pump, a problem that's prevalent with the earlier, original model machines. I learned that the hard way when my old one had the dreaded water pump breakdown.