In the 5 Years I have used Arch (Generally with testing repo enabled) I've only had one breakage (where as I couldn't boot) I can think of that wasn't my fault. (Issue with radeon power management)
As long as you use pacman properly [1][2] and go through your .pacnew files to make sure there's no breaking configuration changes you should be well off, but of course your millage may vary. Any major changes will usually have a news post on the homepage.
[1] One of the major sources of breakages related to improper pacman usage is using -Sy (Update package lists) without updating your system -Su (Combined into -Syu to do both at the same time) before installing something else which make break dependencies.
Basically what happens is you install something that wants a newer version of a library (which other packages which haven't yet been updated to the current versions which are compiled against the newer library) and your new install pulls the new library in with it without updating everything else that depends on said library. Generally -Syu (Update package list and upgrade system) is recommended to be done in all cases where you want to update your package lists.
