I, like a couple of the other reviewers, had a pair of the CX300's for ages (2.5 years and still going strong) and was very impressed, so when I saw that the CX500's were now very cheap (same price as I originally paid for the CX300s), I snapped them up.
At first I was extremely pleased - the sound is brighter and crisper and more punchy than the already excellent CX300's, and are leagues better than other headphones in this price range. I had some slight issues with pressure equalising in my ears when I first got them, but using different and smaller rubber tips sorted that out.
However, I quickly began to notice that the cabling between the headphone and the main cable began to look 'pinched' in places. These 'pinched' parts of the rubber then slowly started to split and the rubber then fell away, exposing the wires. The same process did not happen on the lower part of the cable, which is clearly made of a different type of rubber. I have had the exact same problem with Sony headphones in the past. My other half bought some CX500s on my recommendation, and has had the same happen to him.
The upshot is that while these headphones are still working, they have gotten the point where they need to be replaced within 6 months. Sennheiser have an excellent warranty, and I'm sure will replace them, but I'm pretty disappointed and will not be upgrading again.