I’ve recently done a few upgrades of Debian from Squeeze to Wheezy and it’s a pretty uneventful process.
I recommend following these instructions at HowtoForge.
The only major issue was that Mysql was removed after the upgrade. My data was still there but I had to install mysql-server.
Also you might get an annoying messaging when running PHP that looks something like this.
Unable to load dynamic library '/usr/lib/php5/20100525/suhosin.so'
To fix this run
dpkg -P php5-suhosin
Because I run some stuff using mod_perl, I had to reinstall a bunch of modules, but no big deal.
Other than these issues, it went pretty smooth.
Kudos to the Debian team for releasing another solid Debian version. That’s been my go-to Linux distro for at least 10 years and I’ve never been disappointed.