I think the TH600 is shaping up to be ideal office headphone for me. Like the TH900 it blocks some outside noise, but overall it's not particularly isolating and thus allows me to still hear important stuff. The arc assembly isn't the most robust thing in the world, but the magnesium cups (perhaps erroneously) instill a certain sense of confidence as well.
Sonically it comes pretty close to the TH900 in key respects. So far I'm finding it to be just a bit more polite, a bit more subtle. As I said in my initial impressions, it's almost as if Fostex did this to keep the overall presentation open and clear-sounding since the bass and treble aren't quite as refined as those of the TH900. Spatially the TH600 is slightly more closed sounding, and with regard to imaging it's not quite as easy to zero-in on everything, but separation is still excellent. Presentation on these is overall above average.
The more I listen to the TH600, the more I feel as though it's not a "TH900 lite" so much as something which follows that trajectory but has its own distinct patterns.