I can tell what my buddies have told me but I haven't seen the results yet, as they are both in Monterey and I'm here in L.A.
The areas where the most difference was observed between IP profile and the Gretag profile I made were in the neutrality of an R=G=B gray ramp, where the Gretag profiles were much closer to neutral throughout the tonal range. Another area was that certain bright saturated yellows had a tendency to go green with the IP profiles. Now this phenomenon is something I have certainly seen with many Gretag profiles (check the bright yellow in the highlight of CD on the Photodisc test image), it is really dependent on the paper and the model of printer. To elaborate a little, the green shift appears to have almost disappeared on the 9800 printing through the Epson driver, probably due to much better baseline linearizations. It's still a fairly big problem profiling CMYK contract proofing systems and can be edited out quite easily, but the fact that it occurs in IP profiles means that Colorbyte either doesn't know to look for that defect or that they haven't figured out how to fix it. The other word my friends used to describe the Gretag prints was "cleaner".