I have a Polar F7 HRM.
I got it about a year and a half ago and it has been very useful- I had been trying to make sure I kept to a rough guildline each day I worked out and had used the machines to find I was burning 1000kcals. Now I knew this was in correct, but I didn't realise just how incorrect! The first day I wore the HRM, 1000kcals on the machines = 550 on the HRM. Gutting in a way even though I knew it was an overestimation in the first place.
I do think though, its a mixed blessing. I keep to a 1000kcal on the HRM guideline (which I still believe is exaggerated) but feel worried if I had a bad day where I feel I cannot get that far. I still feel I have to and it just takes me longer and feels more gruelling. 1000kcals can take me 1 hr 30 mins or it can take me 2hrs and 20 mins, depending on how I feel and there are even times I have to stop off halfway to get some energy back, (though press pause on the HRM to do so). Its an obsession and I am not sure if its helpful as much as something which sort of rules my life.
All in all though, I like the HRM I have, its comfortable, the straps have gone a bit yellow but otherwise it looks good, it looks pretty and its pretty bash/drop resistant The only hassel I have is with the chest straps which wear out after a year (and I have 4 I rotate), these are pretty pricey to replace and I find it annoying that they don't last as long as I need them to.
I also like the idea of being able to log stuff online which Polar is supposed to offer. I cannot do this as there is an error with my computers' audio card but it looks like a great idea.
The HRM I have has a few features which I find useful
Primarily heart rate level. I can see how hard I am working out- how much my heart is pumping blood/oxygen to my muscles for them to work via the Heart rate. I can also set my own ranges, low, medium and high as well as min and max.
I can then after a workout see how much of my workout was in my selected range and this helps me see improvements.
I also get a calorie amount. It takes into consideration my age and weight to give me this. I can see a graph for the past 16 sessions to see how the calories varied per day as well as the duration per day. The only irritation I have with this is that if I vary within 100kcals or within say 15 mins it really doesn't show up on the graph. If for example I do 5 days where the calories were 1000, 1090, 950, 1050, 1111, the graph really wont show any real distinction between the amonts which can feel a bit demoralising. (I like to look at graphs and feel I have some power over their structure!)
At the end of the week (sunday midnight) the watch tells me what I have burnt over the week and what my heart rate levels were (eg how much time spent in each level). I can then compare over the past 16 weeks.
If I were to have money as no object, I'd chose a Garmain HRM, purely for the label and the gimmicks, but lets face it, I will never win the lottery so when my current HRM runs dry I will be replacing it with another Polar HRM. They work well, are reliable, have lots of features and look normal to wear.