Im running on NT to and im impressed with version 6. It initally seemed slightly slower, but then theres more feateures etc.
I also when i first upgraded i kept getting cgi no header errors at inconsistent times on both the new v6 test site and my old v5 board which would disappear after a refresh, since rebooting the server its fine though.
I also cam up with adding a simple font style setting to the td tag. and then lose all the font tags within the tables, and then remoing a lot of apostrpophies. This cuts down a lot of code getting sent to the browser. Not sure how it effects browser compatabity, I run a 4 browser only site and its ok on all the big ones.