I have used one with great success, it seem to make my system sound better than any of the other fine preamps I tried, like Hovland, CJ, and Musical Fidelity KW.
Tact Pre Amps 2.2XP
For some reason this post last wound up under "Speakers." Go figure. Anyway, I am thinking of purchasing one of these. There's a unit on Audiogon for $3495. Is it worth the investment?