Although upgrading is getting better, it is nowhere near Debian's level of sophistication. In Debian, even with Sid, upgrades are, generally speaking, pretty painless.
Fedora's sometimes works. The process is getting better, and I haven't tried it in awhile, but, for example, if you use preupgrade, it would fail if you don't have a 500 MB /boot partition. (Not /, you had to have a /boot partition.
I repeat I haven't tried it in probably close to a year, and with the fast moving of Fedora, there's a reasonable chance it's gotten much better, but I tend to doubt it's anywhere close to Debian's, as they had it working quite well 8 years ago. (Although with the general progress of Linux, there's also a reasonable chance that Debian's, which I haven't used on a regular basis for years, is no longer as smooth as it used to be.)
Also, anaconda is nowhere near as robust as a Debian installer--if one package is bad, even if it's some totally unnecessary thing, the entire install will fail.
You can, *sometimes* do a very successful update from, say, Fedora 14 to Fedora 15 just using yum--in this particular version jump though, there will be some major changes, from upstart to systemd for example.
Again, I haven't used Debian on a regular basis for years--rawhide will sometimes be quite good, but other times, every update will break something important. It's not really a rolling release. I wouldn't say breakage is routine, but it's also relatively common, and the newcomer, especially if you're using it for something important, is best off using a release, about a month after it's released.
Fedora will push it out the door before it's ready for prime time--one school of thought is that if, for example, pulseaudio is forced on everyone, it will get into working order more quickly. Now, it more or less works, and seems to be the de facto standard in most distributions.
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