They're now down at less than £15 on Amazon! For the price I don't think you can go wrong. I purchased a pair just under a fortnight ago at £13.49 and was so impressed I then decided to buy a spare set (now at £14.99 but including a pouch). When the second pair arrives I will confirm their value and authenticity. Certainly the pair I am listening to are a step up from the originals and a bargain to my ears.
That said, I have subsequently purchased a pair of Etymotic ER6i 'phones at £60 and discovered that if you are willing to pay that extra and also don't mind earbuds that penetrate deeply into the ear canal, you can find yourself listening with even greater detail, clarity and space. Mind you, I am still unconvinced about the claimed 'enhanced bass' and don't think that I want to spend prolonged periods with the flanged earpiece stuck in my lughole! Personally, I find the Er6i something to get used to and maybe that is wrong - a good pair of earphones, in my opinion, should be readily approachable. However, these 'phones suddenly revealed to me that the CX300 II model, though an excellent product at under £20, has its limitations. Even so, when in the mood, I will switch between the two.