Heart Rate Running

[Riazor Blue]

I'll throw in my experiences aswell. HRM very useful for me and I always wear it. I don't fint the strap annoying, in fact I don't notice it at all. To use an HRM for training I reckon that you need to be at a certain fitness level otherwise the heart rate is much too high for the pace you wish to go. Once you are at a decent fitness level, which I think you probably are, then the pace isn't so bad.
I too have the Polar S410 and am quite content with the functionality etc. The older models, S120, S210, S410 S610 and the S625x have great options for interval training, with recovery according to time, heart rate or distance in the case of the 625. The newer models - RS100 RS200 and RS200SD don't have such good features for intervals, but have something called 'own zone' which is a test to establish your aerobic training zone on a daily basis - for example, if you have run a hard half marathon the day before, you will not be fully recovered so the test will suggest you run at a lower heart rate - all to do with heart rate variability, so they tell me. The speed and distance models are good and give you other options for training, and are pretty good straight out of the box.
Andy is right, you do need to know what your maximum heart rate is (more or less), but also beware that the fitter you get, the lower your max will be, albiet only temporarily. When I'm just running three times per week for fun my HR max is 201 but when near the end of marathon training it will have fallen to around 192. The 220 minus age is a place to start but you will need to measure it yourself or have a test done. The polar HRMs have a feature to predict your HRmax, but for some (like me) it can be way out.
I would suggest an HRM with connection to PC or internet and a good book to get hold of would be the one by Sally Edwards.