I own a pair of 595s, and I am completely satisfied with my purchase. Everything I throw at them sounds wonderful, I especially love strings/femal vocals.
The midrange is definately prominent, and the treble is perfectly refined. I think they balanced the upper ranges perfectly, there is nothing that stands out too much. Everything just meshes.
On the bass side, I can't really complain either. The impact isn't all there like many have said, but I think this is a good thing. I would much rather have bass that is in the background, not overpowering. I find it makes listening to certain genres a lot easier (you can focus on other things). I used to be a basshead, but these headphones changes that. I like the subtle bass I find in the 595s.
The soundstage is impressive, to say the least. When I put the 595s on, I am transported to the artists head, sitting in the first rows of their auditorium. Singers are right in front of you, directing all their vocal love in your area. I can pinpoint where everything is supposed to be on a stage, something I can't do with any speaker setup I have listened to.
I briefly auditioned a pair of 650s, and I noticed that they had more control in all areas. I explain it like this: with 595s you hear the strings on a guitar reverberating, with 650s you hear the pick or fingernails gracing the strings as well as the removal of that pick then the reverberating. I cannot justify the price difference with 650s though, the audio just isn't that much better especially considering you NEED an amp with the 650s.
All in all the 595s are a perfect pair of headphones that don't _require_ an amp and are decently priced. I use these phones everywhere: getting ready in the morning, when cooking, at work and at home... They rarely leave my skull.
Their main drawback? Waiting for them to burn in. I noticed a dramatic difference in their signature as they burned in. I was not liking their sound when I first gave them a listen, but everything got so much warmer as time went on. I now have 200+ hours on them, if not more. Everyday they get better!
This was a long post, but the 595s are simply great!