What is it?
The VAMP is an iPhone case for audiophiles. It has a DAC (digital-to-analog converter) and amplifier that puts the tiny, inexpensive ones in the iPhone to shame. It has a separate audio-out port to bypass the iPhone's volume board altogether, and has its own volume knob. (Yes, a real knob.) And it has a battery built in, so it can charge your phone as well.

Does it work well?
Yep. It's well-made, perhaps a little gaudy but not everyone will agree. Good materials. No hiccups in getting it to work. Sound quality is noticeably improved, especially at high volumes, provided you have the right headphones, the right audio files, and sensitive ears. Sound was slightly cleaner, a little bit more well-balanced, and certainly more capable of driving big, inefficient (but great-sounding) headphones. Has a miniUSB port for charging, which is welcome.

Downsides?
Um, price. Thing costs US$600 at Amazon, which even for audiophiles is a lot. Sound quality is improved, yes, but only after experimenting - I had to use the most expensive, hardest-to-drive headphones in the office to hear any difference. The iPhone supports few lossless audio formats, which are near-essential to get a result that's worth the cost. Case is quite large and heavy. Also, it costs US$600. Almost certainly won't be compatible with the next iPhone, or the one after that. Final point: it costs US$600.

For a much more in-depth look at the VAMP, head on over to Sound + Vision.