I've had a pair of CX500's since just after Christmas (£40 from Play) ... I also have the CX300's as well as countless other ear / headphones (including the pricey PXC 450's for my frequent flights with work).
On first try with the CX500's I would concur with the previous poster that the mid-range is light, nah, its no there !!!
But ...
20 hours or so of running in appears to have loosened things up a bit and now I love these things ... so much so I've had 6 flights so far this year and only taken the 500's ... wayyyyy better all rounders than the massive 450's and in my mind, better noise isolation than the 450's active system.
500's are better than the 300 by a fair way after running in but they have one flaw ... a symmetrical cable arrangement

... the back of the neck approach on the 300's is way more convenient in my opinion.
Volume control's a bit of a gimmick ... although its a slider, its pretty much on / off as there's not much control in between.
All in all, love the 500's ... very much !!
Anyone want to buy a set of PXC 450’s ???