Over this past weekend I decided to upgrade my new install of Debian from the stable 3.0 (r4) release to the testing release. While the stable release is supposed to be "rock solid", it also happens to mostly include programs last updated in 2002, and thus I was finding myself unable to install much of the software that I wanted (R commander for a statistics GUI, Firefox, AbiWord's latest version, etc.). The name of the testing release implies that it's rather unstable, but since packages have to go through some testing before they get to the testing release, it's actually supposed to be quite reliable for the average user.
The upgrade itself was very easy to do. I updated the sources.list file to include the testing release, updated apt.conf (in /etc/apt) to include the line "APT::Cache-Limit "25165824";" (since without some specification of a cache apt-get failed), and then ran "apt-get update" followed by "apt-get dist-upgrade". After a long download, and a few configuration screens of slightly confusing questions, the machine was ready to go.
The primary problem I encountered was on reboot; the machine didn't auto-reboot (failed to finish the shutdown process), and then on restart stopped at the Lilo bootscreen showing just "LI". I discovered that this meant something was wrong with my boot loader/record, so I booted into Linux using one of the stable release 3.0 CDs as a rescue disk. As a side note, I was happy to find that even though the Lilo loader was freezing, I appeared to have access to everything on my system by booting via the rescue disk. I reran lilo, which rewrote the boot loader/record, and since then have been able to boot just fine.
The only other problem I encountered was that printing stopped working. To fix that I upgraded CUPS using apt-get (it had not been upgraded to the testing release's version), put a new PPD file from LinuxPrinting into the proper directory (/usr/share/cups/model/), and then had to restart ("/etc/init.d/cupsys restart") and reconfigure CUPS (using http://localhost:631/admin) to use the new PPD file.
The difference between Debian's stable and testing releases is quite large; I can now use Firefox 1.0, the Gnome desktop environment is much cleaner, and the newer versions of the office-suites (Abiword, Gnumeric) are more professional. The speed is also quite impressive - I'm using an ~3 year old box with an ancient video card, and the system is very responsive. The more I use Debian on this old machine, the more I can see using Debian as the operating system on my primary machine at home (though I need to explore how the Word and PowerPoint replacements interact with my lecture files and work versions of the programs). That all this professional-grade software is open-source and available completely free of charge is extremely appealing.