I have the Cloudlifter. Like Ericlingus said, it does exactly what it says. It's 20ish db's of clean gain (like you'd expect from a Millennia preamp). I've used it live with an Audix OM7 with great results. It brings the Audix up to the output of a normal dynamic.
The slight pain in the ass about the Cloudlifter vs the Fethead is that you need 2 XLR cables with the Cloudlifter, but only one with the Fethead. I've wanted to get a Fethead for awhile now just for that fact alone, but haven't gotten around to it yet.