Pros: Great sound, low price, rollable opamps if you are into that sort of thing
Cons: None realy.... maybe the form factor, since it doesn't match other products so can't be stacked well?
That's a link to my full review, which has become a very long thread.
The M-Stage is a clone of the Lehmann Audio Black Cube Linear, which is a high end headphone amp selling for around $1k. The M-Stage matches or exceeds the real thing in every aspect. It features a socketed opamp in the output stage so you can upgrade or try different things yourself (the Lehmann does not allow this). It is built like a tank, perhaps not the most pretty thing in your audio setup but it does have a certain utilitarian charm. At this moment I think this is one of the best choices out there for people to experience high end sound at a reasonable price.