I've got to say I'm totally disinterested about the "health" side to this. In fact, it very much puts me off the idea of the product.
I don't believe there is a need to monitor 100 different statistics about every bodily function. I think that it's the latest corporatism ploy - measure all this stuff about you so you can feel concerned that you're deficient in this or that obscure statistic. Maybe I'm just too cynical.
If you've ever watched House, where people come in with a headache and worry that it's a brain tumour based on some stuff they read on the internet, you'll know why it's counterproductive to give people so much data. Sure, it's a comedic part of the show, but most of my closest friends from University are doctors and they tell me that it is not very far from that. People are just not informed enough or objective enough to interpret the raw data when it concerns their own bodies.
Data is useless unless you know how to interpret it. It's just numbers until experience and knowledge give it meaning. The human body is an extremely complex machine, and even the most experienced professors of medicine don't fully understand all of the mechanisms it has at its disposal. So giving lots of data to unqualified people (who are very much in a biased position) cannot possibly be a good thing. It will simply increase their own anxiety.
I would compare this to the overprescription of antibiotics. Even doctors (who should have known better) widely prescribed antibiotics as a preventative measure, leading to the build-up of resistant strains. Again, I'm going from second-hand experience, but you wouldn't believe the number of people who specifically ask the doctor for antibiotics when they catch a cold. Every doctors knows that antibiotics treat bacterial infections, not viruses (like the cold), but most of the population don't know or care much about that; to them, antibiotics make you well when you're sick and that's that. Imagine how much more quickly resistant bacteria could build up if you could buy antibiotics over-the-counter! Not to mention the amount of damage people would do to themselves, killing off the perfectly healthy bacteria that live in their guts.
Once people start getting a little information, they're going to see all kinds of 'deficiencies' in their health. Even if they decide that it's not an issue, the idea has been introduced in to their minds. The mind is actually a very malleable tool, and those kinds of suggestions lead people to become hypochondriacs.
Take hypnosis. All hypnosis is self-hypnosis. What happens is the hypnotist is able to exploit your mind's openness to suggestion. Even small ideas, like that your blood pressure is slightly too high, can amplify to become a crisis once anything out-of-the-ordinary happens. Stress does not help anybody, and is a significant risk-factor for all kinds of conditions.
I'm not denying that there are some people who could benefit - diabetics, for instance - but those are narrow cases where a person needs to monitor one or a couple of highly specific parameters. They are given precise instructions for how to interpret that small amount of data. Monitoring a huge variety of parameters and marketing is as a broad-appeal consumer electronics device (which it would inevitably be if it comes from Apple, by virtue of the brand alone) could have to a lot of negative side-effects.
Also, I very much believe this would be the primary function of any iWatch. If you build such a diverse array of sensors in to a product, and hire so many high-profile medical device engineers, it is clearly going to be the main selling point of the product. Besides, what else could you realistically do with a watch? It's too small to really interact with the device much. It's going to be a healthcare-oriented device, that's for sure. The problem I have is that I see it more as a marketing device for the healthcare industry. The whole idea of constantly monitoring and worrying about the intricate workings of your body is something I am totally against.