zero79's right, unstable/sid breaks more often than Testing/Sarge. On the other hand, unstable gets bug fixes, including security bug fixes, first. Testing is guaranteed to be at least ten days behind Unstable, since a package has to stay at least 10 days in Unstable without a bug report before it can move to Testing. The bug that gets a package stuck in unstable often won't bother you.
I run a mix of Testing and Unstable around here. If you get a real stinker from Unstable, you can always apt-get remove the unstable version and re-install the Testing version. If you set it up properly, it's not much trouble to maintain a mixed testing / unstable environment. Here's how I update and upgrade:
# apt-get update
# apt-get install `apt-show-versions -u -b | grep unstable`
# apt-get upgrade
That middle line just upgrades any current unstable packages, and then the (default) testing packages get updated.
The APT Howto
is an unusually readble Debian manual. The mixed environment steps are described (in English, there are lots of other language versions) starting here
Also, sooner or later, you'll probably need to add:
to your /etc/apt/apt.conf file if you are apt-get update'ing multiple distributions. If you aren't totally starved for disk space, I recommend you do it.