It delivers on its promise of clearer audio if the interface lacks a suitable preamp.
For the time being, I've only tried it using a Presonus Revelator io24 audio interface and a Procaster microphone. Without FetAmp, there was a faint hum. I don't need to turn up the gain on Revelator as much anymore. I have incredibly clean audio at the same ultimate audio level now.
If the preamp of your audio interface creates noise, I strongly recommend using a Fet Amp. And Thomann's FetAmp is a solid investment.