These bugs range in severity from "inkscape becoming the default PDF viewer" to "file conflict prevents installing a package" to "libc6 lacks the /lib64 symlink and now no dynamically linked programs on the system can run", and everything in between. Fortunately, the "break the whole system" bugs don't happen often, but still: don't run sid unless you want to help filter out breakage for others by hitting it yourself.
In the mean time, back when I used to live with someone who used Ubuntu as his main system, and when I used to run Ubuntu on my laptop, I had to fix system-crippling bugs at least 3 times.
So, anecdotally, I've had less system-killing problems with Sid than Ubuntu's stable releases.
I've never quite understood people who are running Debian on desktop...
However, I treat Squeeze more as a base to build on than as a complete system. For instance, I work with Ruby via rvm, not native, and use a Firefox Nightly build. In that sense it makes a rock-solid foundation: anything that I'm not responsible for, works. Anything I am responsible for is my call, and can't break the underlying system unless I try quite hard.