I bought these headphones 8 months ago as a replacement for the excellent Sony MDR-EX71SL headphones that after 3 years of daily use, finally had to be laid to rest. I chose the Sennheiser brand due to a good reputation, and for a comparison to the Sony headphones I'd been using.
And as a comparison the CX 300s only provide disappointment in abundance. I originally bought the Sonys for £34.99, and so when I saw these for £29.99 some months ago, I thought they seemed of a comparable value. However, the difference in quality is astonishing!
Firstly the sound quality is not the worst I've heard, but at high volumes the CX 300s show they're a one-trick pony. Bass, bass, more bass, and little else is what they excel at. Treble and mid-ranges are second-best and sound tinny at the side of the deep bass. There's also a hint of distortion at high volume levels. At low to mid volume levels, the CX 300s provide an adequate quality of sound, yet due to the poor noise cancellation, this may not always be an option. The Sonys in comparison although not quite as powerful (read this as a good thing), provided a rich, deep sound at all volume levels.
The build quality is also questionable in the CX 300s. After 6 months of use, they started falling apart, and coupled with the poor sound quality I am now choosing to replace them with the Creative Zen Aurvanas.
The CX 300s also don't offer a short lead, for connecting to command devices, something the Sonys incorporate - a nice, albeit non-essential feature that is lacking here!
In all areas, including sound & build quality, value for money, noise cancellation, the CX 300s are beaten by the far superior Sony MDR-EX71SL headphones.
Having not yet heard the quality of the Creative Aurvanas, I'd go for the Sony MDR-EX71SL headphones - they're much, much better!