Last June and August 2010 I bought REI Sahara Convertibles (Sesame). I love them. The second pair immediately (before wearing) had stitching coming loose in the seat. I had read other reviews of similar experiences. REI replaced them immediately. Now after 8 – 10 months of daily use I see both pairs with the same seat stitching problems. My instinct was to find a seamstress but I tapped REI via chat and explained the situation. She said the pants' stitching should last longer and laid out my options of return for refund or replacement. We found that the Sesame color is being phased out but we found a pair in clearance in VA. I will replace the two pair with two pair of Sand color.
I am amazed that they would take pants back after almost a year of every other day use. I feel guilty. I suspect though that the pants cost them a song.
I suspect others have had the same experience.

