In 30 years of serious riding, so from 7 speed to 12, I can remember 4 chain breaks. Two were on a single, both times a Dura Ace 10 speed chain that was factory installed. Both broke at the install pin, so most likely a mistake in the installation. Caused me to only use the quick links thereafter. Two were SRAM 9 speed chains on our tandem, both had been soaked for quite some time in simple green to clean, which causes small cracks to propogate in the side plates. (Don't ask why it took two breaks for me to figure this out.) Obviously I stopped soaking for a long time in simple green after this. I have chain tools and links in every seat bag on my bikes because I hate the idea of having to call for a ride home, but it's not something I've experienced as a big problem.