Got these a replacement to the CX300 mk1s which I have been using every day as I walk to an from work.
First thing to note is the build quality. A bigger, gold plated jack looks and feels alot sturdier. Its bigger size has however not been a hinderance. The junction reinforcement, where each cable splits off has also been beefed up making these feel much more resilient. Not much has visually changed with the driver unit, it has now got a sloped back but overall looks to be the same size etc.
The biggest difference comes in the sound department (where it matters!). The bass response is much better and is pretty astounding for in-ear headphones at this price. I quite like walking along to the odd dance track and tunes like Mylo - Destroy Rock 'n' Roll (Tom Neville Clean remix) sound excellent. The bass has plenty of weight and presence and the high end is tight & crisp.
I'm not going to comment on how well these head phones fit as that is a completely personal interaction, they either will or they wont. There are three different sized rubber ear pieces included, so hopefully one pair will work. There is also a soft sprung clasp pouch included, for stowing the headphones when no in use. This is a welcome extra but I've reverted back to my usual hard shelled tin as it offers a bit more protection whilst bouncing round my bag.
To summarise, if you already own CX300s, the mark twos are a worthwhile upgrade. If you are in the market for in-ear headphones, these should not be overlooked. Top Quality Sound and Build for next to nothing.
**** ONE YEAR EDIT ****
Well after a year of daily use (hour walk to and from work) a connection in the left ear has gone. Resulting in only faintly audible levels coming from the left hand side. There's a two year warranty so it should be covered.