The last time my X broke was a couple years back. Ubuntu pushed a defective update. Before that, the last time something like that happened was when I was using Debian Sid.
Breaking X is something you expect with Sid. And if you are running it, you re supposed to be able to fix it and submit a patch.
If you are breaking X, you are doing something wrong.
I think it probably depends on the distro, I've had quite a few systems break on Arch because I didn't read update warnings on the Arch Linux website after they had pushed a bad update.
Tangentially that is why I moved to xubuntu from Arch, though I'm sure Arch is a bit nicer with regards to headaches now.
Fedora likes to break frequently though I don't know if packages as big as X are likely to fall through the cracks.
Both Arch and, to a lesser extent, Fedora are aimed at more advanced users who want faster updates and newer technology over stability.
I think this is a great compromise, but it does mean you may have some issues with updates. That said, I have not had any issues on Fedora that weren't my doing.
I've been using Fedora for about a year. Earlier, I used OS X for about the same amount of time and I did have problems that weren't entirely my fault, largely with Java and Eclipse. Since all I was doing during that time was simple Java development for school, there just wasn't anything else that could have gone wrong.
Breaking X is something you expect with Sid. And if you are running it, you re supposed to be able to fix it and submit a patch.
If you are breaking X, you are doing something wrong.