I ended up buying one of these, and several months later it's still working fine. I don't know how it compares to other frequency division detectors, as I haven't tried any others.
Used in conjunction with my heterodyne detector, it seems to pick up whatever the heterodyne detector does, possibly from a greater distance. I find the sound of it unpleasant compared to the heterodyne, but I think that's probably normal for frequency division. My intention is to use it to detect bats out of the range of the current setting on the heterodyne, to help me tune it in.
I particularly don't like the seemingly random noises it makes in the wind. Wind on the heterodyne at least sounds a bit like wind. This could also be a typical frequency division problem.
It has no volume control, just an on/off switch. The case is all black, and appears to be 3D printed, which gives it a non reflective texture. That explains the featureless photos that were on the website at the time, but they've been replaced by better ones. I added markings with a white pen to indicate the off and on positions, and to help me find the switch in the dark. The surface texture isn't good for writing on.
The microphone is on one side up the end, and the speaker is on the other side, up the other end. This means it works best held up in front of you towards the bat, which can be awkward. The volume isn't that loud, so I prefer to have the speaker facing me.
I've used the headphone jack to make a recording early on, and from memory the recording was ok. We're just coming out of the cold weather here, so there haven't been many bats around to try it on.
The tiny size means I can take it anywhere. I think it could benefit from some kind of clip or lanyard so you don't have to hold it, so I'll probably add one.
It would cost me $75 now, due to changes in the exchange rate, but still worth it, I think.