I've hit with the new stick for 4-6 hours and have used the IG version (18x20) for about 18 months.
Graphene speed is stiffer than the previous versions, but more comfortable. There are less vibrations and better torsional stability which give it a nice solid feel. I thought the response was crisp. Sweet spot is bigger and a different shape (matching the different head shape).
Overall I think it's a great update/upgrade. There isn't anything I really prefer in the old version and found the new version easy to adjust to. Some other people at my club though didn't like it as much as the older version.