apt is good software. I just upgraded my server from stable (Sarge) to testing (Etch). The trick is my server is 5000 miles away from me. And I will never have physical access to it, so if something goes wrong, I'm screwed.
But Debian did me proud. Run apt-get dist-upgrade, watch 150 packages go by, watch the new kernel install, reboot, say a little prayer, and I'm back. Some scariness from the kernel install script:
I repeat: you have to reboot in order for the modules file to be created correctly. Until you reboot, it may be impossible to load some modules. Reboot as soon as this install is finished (Do not reboot right now, since you may not be able to boot back up until installation is over, but boot immediately after). I can not stress that too much. You need to reboot soon.
I've done major system upgrades like this three times now with Debian, and everytime it's worked. That's pretty impressive.