I've had the GR07 CE for a few months.. I've owned both the MKI & MKII in the past (awhile ago.. but I had each of them for a long time). To my ears, the CE is the best of the three versions.. it possesses a deeper stage, better dynamics, and the upper mids are much smoother and less peaky (the upper mid spike is noticeably less present on the CE than it was on the MKI or MKII). It's about as close to the HD650 (latest silver screened-driver) signature as I've heard.
That's good to hear esp since I believe the MK1 price was closer to $200. Although these resembling the HD650 might be why I didn't like the MK1 that much. I'll find out for myself on Friday.