I got my XS today - first impressions - this is probably the most comfortable on ear I have tried. The M-100 was too small for me, and I found it very uncomfortable (didn't try the XL pads). The M-80 clamped too hard and was pretty uncomfortable after a short period... I have been wearing the XS all afternoon and I still find them comfortable.
Isolation -- not that great. I can hear myself type while wearing them, I can hear my mouse click no problem... I was really hoping I would be able to wear these on a plane (and cut out some noise) but I would probably have to crank the volume way up
Sound -- very good. I found the M-100 to be too bass heavy, The XS sounds very good. Slightly warm, bass isn't muddy at all. Fairly balanced sound, the treble does seem to roll off a tiny bit.
Soundstage - pretty small. I just did a quick switch from the XS to HD800 --- HUGE difference.
Overall I am VERY happy with them. Comfortable, good sounding, lightweight, very portable. My only wish would be a bit more isolation.