I had two sets of QC20i break on me in the exact same way (the glue on one of the earbuds failed and the earbud just split open). First was in warranty and they replaced it no questions asked. Second was just out of warranty and they wanted £150 for a replacement set.

I'm not paying £150 for something that's failed twice in the exact same way, so I bought some Shure monitors instead. The passive reduction on these is as good as, maybe better than, the active cancellation of the Boses, and sound quality is way better.