Sid is quite stable but things will occasionally break (once or twice a year). Never so bad that it needed a rebuild (touch wood) but enough to be scary if you don't know enough to keep a cool head about it.

Latest example is some nice person breaking udev around Christmas causing it not to mount LVM partitions during boot. Luckily Linux does the right thing and drops you to an emergency shell and it's very easy to mount things by hand and resume boot. It did make my heart beat faster for a few minutes though! ;) This was the worst breakage for several years to give some perspective.

Also, best not to upgrade the (Sid) system for a few months after a new Stable release - that's when the floodgates (from Experimental) open and issues are often seen.

Overall, it is easily worth it for me to run Sid and get the latest software versions. I always run Stable on servers though.