I got one of these watches a couple of years ago, a slightly different Mio model, but overall I have been pleased at how well it works with respect to the heart monitor aspect. I always wanted to know my heart rate when walking at work, but the chest strap thing was just not workable, so I was excited to find this watch. It really does give a accurate reading as I have checked it often against commercial teadmills with the chest strap. I have used it walking outdoors at lunch at work, as well as on a treadmill at work and just walking around the building. It really helps me keep in the "zone" that I'm shooting for, and is great because I have never been able to successfully take my own pulse using the fingers against the neck or on the wrist.
It usually is easier to get readings in the summer months than winter because up here in the north one's skin and fingers can be very dry and it requires some moisture both on your arm and your fingers to be able to get a reading. Often I can just breathe on my fingers and that provides enough moisture to make the reading possible. I was originally very skeptical that this could work at all, since nearly everything that measured a pulse used that chest strap thingy, but this watch really does do the job.
While I have no problems taking a reading while walking on the treadmill or outside, I do agree that a lot of body movement can make it more difficult to hold one's fingers on the sensors. It usually takes me about two or so seconds to get a reading, depending on the circumstances.
Unlike the chest strap stuff, you can take a reading on this watch at any time, so if you are climbing the stairs at work to get some exercise, you can take a reading when you get to the top to see how hard your heart has had to work to get you there. I even used it in a meeting once when I got really stressed out and it showed measurably by an accelerated pulse as measured by the Mio! Imagine the fun you could have while stuck in a massive traffic jam, watching your pulse soar into the "red zone" just sitting in your car! Or taking your pulse at the airport after you have been told your flight has been cancelled. Seriously, though, I have found that my pulse and my stress levels are related in that high stress always gives me a faster pulse, even at rest. When I see that, I try to take action to lower my stress level and thus my pulse. Sometimes you can be stressed out and not really realize it until you see your pulse racing when it should not be. Your heart is a lot smarter than you give it credit for! So for me I have a "low pulse" goal that I strive for as well, and when I am near it, usually at home relaxing, I know my stress is under control and I am doing well. I have found with this watch that the lower the pulse rate I get, the happier I am at that particular time more often than not. To me the Mio proves that stress is more than just in one's mind. It has a physiological affect on your body as well, almost all negative!
Considering the modest cost of the unit, combined with not having to worry about bulky and expensive chest straps, especially at work, and that you can always have it handy to use, I'd say I'm happy with the watch. The only thing I would add to this would be what my other watch I own has: automatic setting by the NIST atomic clocks (over the air) each day to keep the time perfect.