I just finished reading Andrew Robinson's review of the B&W C5 headphones. As usual, it's another of his well researched and thought out reviews. I, too, have a pair of B&W C5 headphones for evaluation, and while I don't disagree with Andrews overall findings I would like to emphasize several important details about the C5's real-world performance.
Andrew found that the harmonic balance and bass response of the C5 in-ear headphones changed depending on the fit. For me the sound changed RADICALLY depending on the fit. When too firmly entrenched in my ears with no gaps, the C5 produced way too much bass for my tastes. If I positioned the phones so they weren't fitting as well as they could, bass response would drop precipitously. Obtaining a fit where the bass response was the most natural and linear was difficult - too tight meant too much bass, while too loose meant no bass at all. Finding the right fit for balanced bass was, for me, a very transitory thing. And yes, I tried all the tips...
While not the most gym-unfriendly in-ear phones I've ever used (that prize goes to the Apple stock phones which won't stay in place while I'm riding an exercise bike or running on a treadmill for more than ten seconds) the C5s still proved to be less than ideal in any situation where I needed to move around. Andrew mentioned the too-short cord in his review, which if you wear your iPod on your belt, needs to be at least a foot longer, unless you're under four feet tall.
After a couple of weeks with the C5 I've concluded that for me it is very much a stay-at-home headphone, so the fit and cord-length issues aren't a problem. But given the trends toward mobile devices, can an in-ear headphone that needs to stay at home for optimum performance be considered a success?
Perhaps if B&W added a few more ear-tips (such as a triple or double flange) the C5's fit could be more universal and remain more stable during activities. Also a longer cord is mandatory for use by a normal-sized human.
Your own experience with the C5 phones could be different, since only a small percentage of people have narrow ear canals such as mine, but I can't unconditionally recommend the C5's as they are currently configured unless you make sure they fit you comfortably, will not shift during your usual daily headphoned activities, and you wear your MP3 player well above the belt.