I bought a 6700 equipped bike in early april. After less than 2,000 miles, the rear shifting was complete crap (very heavy effort to downshift, like it was stuck). The bike came with Jagwire.

I put new Jagwire housings and cables on it. I thought about going with some higher end stuff, but I can re-cable it with Jagwire for under $20. Shifts great again, we'll see how long it lasts. The orignal cable job wasn't great with a tight bend at the rear and extra cable up front, so I hope this time will last longer.

I did have quite a few rain days on the bike that probably caused some issues, but my 5600 equipped bike has over twice the miles with a bunch of rain days and it still shifts like new.