Excellent.

Author's Rating:

Pros: excellent bass, very sweet mids, tiny drivers, comfortable fit, great price

Cons: personally hate the asymmetrical cord, highs sometimes get drowned out by bass and mids

The Bottom Line: 
The Sennheiser CX300 is significantly cheaper than most hi-fi in-ear earphones yet provide superior sound.

Author's Review
Sennheiser does it once again. Relatively inexpensive products that deliver equal-to-superior sound quality to some of the competing products that are often double the price.

As a comparison point, I will compare the Sennheiser CX300 to the Shure E2c.

1. COMFORT
The Shure E2c has an odd fit, with the cord folding around your ear. The CX300, like all in-ear earphones, come with three different sizes, depending on your ear size. They fit right in and will stay there until you tug it out. The E2c takes some time to get it to fit, but CX300s fit right away.

2. PRICE
The Sennies obviously win. The CX300 are about 20% cheaper.

3. SOUND
The CX300 produces massive bass response, sometimes a little too much (if you are a purist). Personally, I love the bass because I listen to a lot of bass-heavy Eastern instrumental music. The E2c also has nice bass response, but not as much as the CX300.

The CX300s also win in the mid-range sound category. Somehow, Sennheiser figured out how to make mids sound very sweet with these tiny sound drivers. If you listen to a solo alto singer using these CX300s, you'll be in music heaven.

The highs on the CX300 are kind of drowned out by the heavy bass and good mids. A flute will sound nicer on the E2c, but male voices and female alto singers will sound better on the CX300.

Overall, I'd say the sound quality of the Sennheiser CX300 is superb for the price. They easily match the quality of the Shure E2c, and even surpasses them in many aspects. Sennheiser has always engineered the best in sound equipment, and the CX300s would be the BEST value if you're not looking to pay over $100 for earphones by Etymotic Research or Shure E3c or beyond.

If you're a bass and sweet-mids lover, go for the CX300. If you're a purist, go for the Shures. If you're a high-end, music savvy musician, go for Etymotic Research. Sennheiser still brings excellence to their sound.

By the way, I live in Chicago and these CX300s block out the majority of the outside world's noise. I can sit in the trains and be unperturbed from the excessive noise. Be warned, however, that you won't hear car horns if you happen to walk across a street when you're not supposed to!! (happened to me!)