There have been reports on the forums of people having difficulty upgrading from 6.06 (Dapper), but I’ve had no problems here. (I used the CD-ROM to upgrade.) I’m in the middle of some consulting work so can’t upgrade the other machine, but I’ll update this when I complete that install.
Update: the upgrade on my Dell Inspiron 9300 went fine. I got one error during the “apt-get dist-upgrade” step, but running it a second time retrieved the remainder of the packages. Power management is much improved over Dapper.