I can't vouch for how well these will hold up over time, but it's worth noting that the cords are thin, so I wouldn't count on these withstanding too many tugs -- still, I haven't had any problems in the two weeks I've used them. It's also worth mentioning that the 770X has a straight plug, not an L-shaped plug that many people considerable more durable.
The other small strike against this model is that it doesn't have an integrated microphone and remote for making cell phone calls, and there's no step-up version of the same earphone that offers this feature (the $65 NE-700M, an earlier model, has the microphone).
Missing microphone aside, these are a very solid value at $49. I tested them with a variety of music including The National, Usher, Foo Fighters, and Elvis Costello, as well as our CNET headphone test (an ensemble of lossless tracks put together by CNET contributor Steve Guttenberg). You're not going to get the incredible detail that you get from much more expensive earphones and headphones, but overall the NE-770X is pleasant and punchy, and the sound doesn't feel stuck inside your head (it sounds fairly open).
Bottom line: It's hard to expect too much more for $49. Recommended.