A few weeks ago my old CX 300 broke ;( ,which I had for almost 3 years, and wanted to buy another of them. However when i looked on play I saw that the CX 300 were £17.99 and then I thought great deal I'll them but then I the CX 300 mark II.. After about a week of thinking (no joke, I'm still in school and don't have a job) I decide to get the CX 300 mark II. Three days past and the headphones arrived, very quick deilvers, and when i opened the package i was amazed at the improvments over the previous model. First off the headphone jack that goes into your ipod or whatever, had been made so that it won't just become lose and come out and break like in the CX 300 (this is how mine broke after the glue wore away). Senhiser has also made improvments to the length of the cable (YAY) this means that when you turn your head they won't fallout of your ears! the cable also feels much stronger and more expensive than the CX 300's. The sound quailtity has also been improved, mainly the bass.
Overall if you have the extra £12 I think the its well worth getting the mark II, but if you don't the CX 300 are good aswell...but not as good :)