When I first got my 650s, they were replacing a beautiful pair of 580s. I was so psyched... plugged them in... put in a disc... ugh. They did not sound good at all. I thought, OMG, what have I done. I know my 580s took a long time to burn in, but I guess I was so conditioned to their wonderful sound- it was hard to fathom that the 650 would ever match it.
But after a very lengthy break-in period, they are excellent. You will note a marked improvement after a fairly short period, but it does take a good long time for them to really loosen up and come alive. The midrange has a nice balanced fullness as compared to the 580s, and the highs are more articulate as well. When I first tried them, though, they were muddy compared to the 580s. Scary.
I later added an Equinox cable courtesy of SeanH, and that really completes them. Since then I have bought sr225s, dt880s, k501s, and sold them all eventually, retaining the 650s.