Summary: I owned a few other Sennheiser in-ear headphones and have been quite happy with the latest one (MM70i). The reason I purchased the CX880i is because the MM70i's iPhone/iPad plug got broken, and according to the very limited direct comparisons of the two models online, the CX880i is practically the same as the MM70i's, with an L-shaped plug instead of the straight one in the MM's.
Summary: I bought these thinking they would isolate the background noise whilst travelling in London, especially on the London Underground. Unfortunately they don't really do this because the ear piece jackets are made of just thin rubber rather than foam rubber which would block more noise. Otherwise the sound quality was brilliant. I'm more likely to buy Shure again as their jackets are made of foam rubber and really isolate background noise extremely well.
Summary: This is the third pair of Sennheiser headphones I've owned and like the other two pairs the sound quality is excellent and I can't fault it. I bought these as I own an iPhone and wanted the option to use them for making calls as well as being able to listen to music/films through quality headphones. They do exactly what you'd expect them to do and the microphone works very well. The reasons for not scoring these headphones more highly are two-fold.