I use this with my electrovoice RE20 mic.
It's really a useful little fellow. When I had bought a DBX286s, I thought maybe I can do away without it since there's already a preamp + audio interface (UAD) in my chain. However, I was mistaken.
The gain from DBX286s was actually noisier than from Fethead. Which was totally unexpected to me. Having that said, of course, it could be even better!
Don't overlook it just because you think you will find a better budget gain source for those hard to drive mics.
Also, there's a lot of comments suggesting where to place Fethead. I tried multiple variations - inserting it directly into mic, preamp, audio interface, separating it via cables.
No noticeable difference, so go crazy.