I agree about the sound. In particular, the mids and highs are clean and clear without any deemphasis of the low end. The sound is dynamic, emerging from a black background.
Downsides? Cost has already been mentioned. Also, having an extra chassis for the power supply eats rack space, and after a while I tire of waiting for the built-in delay circuit when switching sources, etc.
By the way, I have stayed with the stock tubes, reasonably happily. I'm curious to read about others' experiences with different tubes.