Just thought I'd chime in on this post:
As a fitness professional, I've owned several Polar HRM's over the years. I use them faithfully in my indoor cycling classes. They've been an invaluable tool for maximizing my training time.
Polar certainly has a vast product line, and I personally think that they are a reputable, high quality brand. I will admit that I do get overwhelmed when I look at their current website; if I weren't an existing customer, I wouldn't know where to start and would probably look at other brands. For basic fitness & group exercise classes, I recommend the Polar F4 or F6 since they monitor & display your heart rate, offer tracking, and allow you to set your own heart rate limits. No matter what brand or model you decide upon, I definitely recommend getting a HRM that allows you to set your own heart rate limits and one that eliminates cross-talk (i.e. picking up the heart rate of someone close by who also has a HRM). I have owned the Polar M52, A5, and F6 models; the M52 was my favorite, but they don't make it any more. The A5 was too basic for me and didn't have some of the features that I wanted. I like the F6 because it has the Polarlink upload feature, has a comprehensive diary feature, and has the ability to manually set your heart rate limits. There are aspects of the F6 that I find annoying, but for me it's a pretty comprehensive tool.
As for determining your own heart rate training zones:
Aside from doing a true fitness test, I recommend determining you own heart rate zones using the Karvonean Formula. There are several on-line calculators these days that will calculate your heart rate training zones for you, given only your age & true resting heart rate. you can determine your true resting heart rate by counting your pulse for 1 minute when you wake up in the morning, before getting out of bed or stirring around. you could even wear your HRM to bed and check the pulse when you wake up. take the average over 3 days and you have your starting true resting heart rate. You'll want to re-establish your baseline every 3-4 months, because as your fitness level improves, your resting heart rate should decrease.
Here is a decent Karvonean calculator that will do all the math for you:
http://www.briancalkins.com/HeartRate.htm
If anyone is interested in learning more about the science behind the Karvonean formula, just PM me. **edited: actually, i remember that i posted about the heart rate training topic a few years ago that has the information that i am offering to PM -
http://www.thecathenation.com/forum/showthread.php?t=206945 **
Oh, more more thing about HRM's:
Make sure you know how to replace the watch battery. Polar HRMs have to go out to the distribution center to have the battery replaced. Replacing the battery yourself voids the warranty and will likely destroy the watch (I speak from personal experience). One of my user guides explicitly stated that the watch has to be sent back to polar for replacement, and that if you use a standard watch battery, the watch won't work. It is true - lol. The good thing is that 1) it does take a long time for the battery to die (took mine about 3 years, using frequently) and 2) the price of battery replacement has decreased drastically. Replacement for mine cost about $18, including shipping & tax.
Sorry for the long post, but I hope it is helpful in some way!
Enjoy your search for a HRM!!