Some specifics: The box itself is very light, easy to operate, and about the size of a box of matches, or the video screen of an iPod Classic. It's also really light.
The headphones are robust, with three sizes of earplug, which is useful.
This machine works - I have to say, it's great to be able to swim with audio. But you're reading this trying to find out what the downside is, so I'll tell you - you will spend time fiddling with the earphones. I have reasonably large ears - not freakshow, but I'm a tall guy and they are in proportion to my head, and they took three swims to get in properly. And in so doing, were a little uncomfortable. Now I'm used to them, and have the correct ear plug size on them, I am loving the sound, which is a little better than radio - you won't be getting amazing bass response here unless you ram those suckers so far in the tips come off (this happened to me at one point - tweezers and a lot of calm!)
With all this said, those are the only downside. Light, easy to use, and a lot of fun. And, once you're practiced at using them, great to show off in the pool with. I have already had two people ask me where I got them from, and it's only been a week or so. Recommended.