apt-get upgrade only does "simple" upgrade, not requiring removal or addition of software packages. This *WILL NOT* take you from "Edgy to Feisty". This should not "break" things.
apt-get dist-upgrade does removal and addition of software to transition to a new distribution. This *WILL* take you from "Edgy to Feisty". This could "break" things.
Now, understand, Ubuntu releases based on time, buggy or not. This is my problem with Ubuntu. I understand Debian releases with bugs as well. There is a difference in release quality as well.
Ubuntu does the "deadline thing". It makes, sometimes, foolish choices about a RC bugs. It hurts when upgrades happen. It hurts when a few squeaky minority complain loudly and make life hell on the forums and MLs. They don't follow the prescribed methods... they have 3rd party *UBUNTU VERSION SPECIFIC* applications that break upon upgrade from dist to dist. Backports hurt many times, response is dismal, it hurts things. They want the problems with 3rd party stuff fixed on the release date.
Debian, has many of the same problems, 3rd party apps are a bane as well. But since Debian has a HUGE repository and many of the apps that are "3rd party" in Ubuntu, in Debian are buildd-built in the Debian archive, relieving much of the pressure. If you understand *WHY* this is a huge difference, then you understand why I use Debian.
Yes, I have Ubuntu on the machine in the living room, mainly I don't use it. They wife and daughter don't "have the time or patience" to deal with things that need massaging. Currently media playing on that machine is broken due to a security update. Mplayer is bustified, totem-gstreamer is also bustified. All due to codecs issues. But since 7.04 is so close, it hasn't been fixed for a while. Yes, I've done the work arounds... but being that I use an EMU10K1 sound card in that machine... it's bustified.
My Debian machine isn't bustified. I did have an issue with the stuff being broken for about 3 days. Not anymore.