Apart from diabetics it would also be useful for the glucose-willpower debate.

As a diabetic, I would dearly love to have something like this. I read Google was working on a contact lens blood glucose monitor with Novartis.

If you could build the monitoring part of it - reliable, accurate, near-real-time and not too invasive - you would have an instantly revolutionary product, quite apart from piping the info to a new sensory channel.

This goes beyond the general usefulness and "cool factor" of many of the other suggestions into very useful, potentially life-saving, and helps fill a purpose which we currently do fairly inefficiently.