I wouldn't expect this to weigh more than 100 grams. In comparison, a Rolex Sea Dweller weighs 220 grams (http://en.wikipedia.org/wiki/Rolex_Sea_Dweller).
For me this would be useful when going to the gym, I could listen to music, see urgent emails and if it had GPS then track bike rides and runs. If it had a heart rate monitor then perfect. I don't like the fact it has to connect to your phone, standalone would be preferred.
It's not that much different from using the iPod Nano as a watch - http://www.engadget.com/2010/11/18/ipod-nano-review-as-a-wat...
But regardless of the size of this, have you been to a watch shop recently? Some of the sizes of the watches are ridiculous and I presume they sell... so this might do just fine!