This unit provides a solid boost to the line level of a microphone, with no perceivable added noise.
I'm using it with a passive ribbon mic that has a very low output level. This is fine when mic'ing an amplifier, but was too low for many applications like vocals or acoustic guitar. The FetHead has provided exactly what I needed, giving me plenty of level without having to drive the preamp too hard.
I went for the FetHead Filter (which has a subtle 6db/oct roll-off below 220 Hz) as I found I was putting a high-pass filter on most tracks where I used the ribbon mic. The filter takes out any rumble quite nicely and simplifies mixing.
The only little imperfection was that the Triton Audio badge had fallen off the FetHead in transit. But it was purely cosmetic and I just glued it back on. The unit was fine otherwise!