If you're a frequent traveller or you find over-ear headphones uncomfortable, then in-ear headphones offer great sound for a relatively small price. Sennheiser has always been the "go-to" brand for portable headphones, and its original CX-300 was one of the most popular in-ear models on the market.
Sennheiser has upgraded the headphones with the promise of "greater clarity and improved dynamics" and features high-quality dynamic speaker systems. None of the fancy "custom-tuned armature drivers" of more expensive sets but based on our listening tests, they are indeed "high quality". Build quality is quite good for the asking price; but while you only get plastic housings, you need to pay a lot more to get better construction than this. The cable has a tendency to tangle, as well.
The headphones offer an asymmetrical cable, which means that the right ear bud dangles off the right, and while this is meant to increase listener comfort it also means that the left ear bud is more likely to fail with repeated strain. Pull on the cord and it tugs directly on your left ear.
The Sennheisers come with three sets of plugs so should fit most people's ears — unfortunately, there's no foam ear buds included. These would offer a greater degree of isolation from the environment, but for that you'll need to invest in something like the Monster Jamz or even the big daddy Ultimate Ears 700.
Sound quality is excellent for the price, and while Sennheiser says the headphones offer powerful, bass-driven stereo sound it's not as overwhelmingly bassy as that might sound. While a bit more money spent on the Monster headphones will give you tighter bass response and more kick they're a little more fizzy in the upper register.
Given the weighty sound of the CX300's they're well suited to rock and dance, but they impart a warmth that is also good with jazz. But that's not to say they're not detailed as they unearth parts of the music some other headphones can obscure. But most important of all, they make music fun.