I haven't heard of any breaking, and a lot of riders in my club use them.
Here is a review and some reader comments on
www.cyclingnews.com:
http://www.cyclingnews.com/tech/?id=2002/reviews/keywin
http://www.cyclingnews.com/tech/?id=2002/letters/keywin
I ride Looks, but would consider Keywins should I be upgrading pedals in the future.