I had (past tense) a set of Ksyrium SL's. I broke two spokes within the first 2,000 miles. (I'm ~170 pounds, ride reasonably smooth roads for the most part). Sold 'em. I'm back on hand-builts. My last set of hand-built wheels have about 30k miles on them and have never seen a spoke wrench let alone a replacement spoke. I'll take the 100 gram weight penalty of knowing my wheel isn't going to break for a third time.
Apparently I was in the minority of those with Ksyrium issues, because most people seem to rave about how strong they are. I dunno.