I got the Shure E2c sound-isolating earphones yesterday from Amazon for about $60. Almost all of the major reviews I could find described them as excellent earphones for both their sound-isolating properties and their audio quality. I've only used them for about a day so far, but here's a sort of review of how well they live up to the high standards I started out with.
Standard earbuds are disk-like speakers that sit on the outside of your ear, not extending into the ear canal at all. Some earbuds have a little protrusion that goes a little bit into the ear canal, but only a few millimeters. In contrast, sound-isolating earphones go pretty deep into the ear canal, up to about a centimeter. They block background noise passively, like earplugs do: by having a rubber or foam plug that stays in the ear canal. The advantages of this are that they block sound very effectively without requiring batteries like noise-cancelling headphones do, and that they won't come out when you run because they're deep in your ear. The disadvantages are that it can be hard or painful to put them in if you don't know how to do it, they collect earwax pretty easily, and (this is also an advantage) they block outside sound very effectively, so you can hardly hear someone who's standing next to you, even if you're not actively playing music.
The Shures are excellent at sound isolation. In a loud room like the school cafeteria,
the roar is literally quieter than a whisper with the earphones on. This lets you play music at low volumes and preserve your hearing, since you don't have to drown out any sound.
In terms of sound quality, the treble is amazingly crisp, and that really adds to the enjoyment of the music. The bass is very accurate (not artificially high, nor too low) when you're using a decent sound source like a computer or a good MP3 player, but with my poor-quality player, there's very little bass, so that detracts from the experience a little.
Finally, comfort. At first, it's very painful to put these earphones in. You try to push but they just don't budge, and when I say it's painful, I really mean it. But what really helps is to wet the tips a little, which makes them go in easily and painlessly as well as making a better sound-isolating seal.
So overall, I'd say these earphones are quite good—of course they're a huge step up from standard iPod/Creative earbuds, but their sound quality is actually better than even a pair of $100 non-sound-isolating Bose earphones we have. And their sound isolation just makes it even better.
Update 2007-12-20 2:20am: Shure provides three kinds of tips for the earphones: clear hard-ish PVC ones, black soft rubber ones, and orange foam ones. I had been using the black rubber ones and I always had to push them uncomfortably deep into my ears to get good isolation and sound quality, but I just tried the orange foam ones, and they're much better. They're very comfortable, I don't have to put them into my ears very far at all, and they provide even better sound isolation. The only problem is that they're not as durable as the others since they're foam, so I'll have to replace them in a few months. Also, they take longer to remove. Overall though, they make the earphones nearly perfect.