I've only broke one wrench, it was a new Craftsman Pro wrench the first time I used it. I'm not a Craftsman basher, I actually think their the best bang for the buck (plus it's American made), but I was amazed it happened. It broke just like the picture above in Mickey O's post.
Sears was great about it, I grabbed a new one of the shelf and exchanged it with no hassle. What's funny is I bought the set to have a step up from my "regular" Craftsman wrenches which never had an issue. I might have just got a bad one, but I still think the Craftsman Pro is a great value, and haven't had any issues since. I'm certainly not going to throw down $1200 or so for a full set of Snap On wrenches.
I was using the open wrench on the flat part of a tie rod to hold it while I was tightening the end nut, and I wasn't really putting that much stress on it, but it was a small wrench (like 12mm).