lhunter,
Thank you for the positive feedback! The MC team is working hard to make the upgrade as seamless as possible. You should increased speed and performance and alot of new functionality and design flexibility. In 30 days or so you should see a jump in traffic with the new .html catalog generator.
Call volumes with clients enquiring about 4.0 has increased more than we had anticipated. We had anticipated that call volume would double... in reality it has quadrupled!
Not to worry, we have increased everyone's hours during the migration process and (except for extreme spikes during some parts of the day) tech support should be as responsive as always).
We are also working hard to correct any bugs clients find. We did extensive testing... but there are always items clients find from unique store uses that we did not anticipate.
Although migrations have just begun, we have received very good feedback from clients and hope the migration process continues to become streamlined (we have to migrate one store at a time) and we have a team that checks design, check out etc... so it's time consuming but worth it.
Thanks for being patient if you do have slightly than longer tech support hold times... it won't last for more than 3 or 4 weeks.
And, as always, please express your needs in the forums... we are monitoring them hourly!

Thanks again for your positive comments... it's what makes all of this worthwhile!