I got it

ran a super grimy AEKII keyset (dyesub PBT) through it in three batches.
Spoiler alert: it works great.
A few observations:
First thing: the reservoir is much smaller than you’d think from the outer dimensions of the thing, though the spec isn’t lying; you could probably fit 3L in there if you filled it nearly to the brim. I used 1.55 L.
Second: the heater works, but it’s ridiculously slow. Taking the solution from 15 to 40 degrees C took so long (at 100W it should theoretically take 18 minutes if there were zero loss; it takes more like 25-30 minutes) that I’m probably going to pre-heat the solution in the future.
Third: when the ultrasonic transducer is on, it makes a loud, high pitched screetching noise. I figured it would be a quiet hum, but it’s really not.
For the solution, since I was just cleaning keycaps, I went with tap water and a commercial ultrasonic cleaning fluid. The fluid is just from a hardware store and is labeled as *not* suitable for cleaning jewelry or eyeglasses. It said to dilute it in water 1:10 to 1:20, but I went with 1:30 (50 ml to 1.5L of tap water) because I wasn’t sure what to expect. Oh, and I reused the same solution for the three batches. I am sure you can keep using one solution for many batches.
Temperature wise I went with 35-40 degrees C but I will try with different temperatures in the future.
Finally, I did experiment a bit with the duration, and 4 minutes seemed just right for the disgusting AEKII caps. I did stir the solution while it ran, and the results were almost perfect.
YMMV of course, and I would love to hear how others use theirs.
Before:

- 8020414C-534C-4812-BC56-FA279930DBB8.jpeg (410.62 KiB) Viewed 1371 times
After (note: I haven’t cleaned the case at all):

- B07527F4-890A-43D2-8B8A-06E681F55876.jpeg (236.21 KiB) Viewed 1371 times