Danny Dreyer, author of Chi-Running is about the only running guru I know of that is against Heart Rate Monitors. He thinks it's best to just listen to your body. That by paying attention to your body and not a bunch of gadgets you can tell more about your body than any gadget can. People have off days and if you paid attention only to a HRM you might over do it by trying to keep your Heart Rate in the right zone. as opposed to just running at a pace that feels good.
However, I have a HRM and I'm really happy with it. My main concern is that when I'm running and day dreaming I have a tendency to slow down. I also wanted to make sure I would push myself but not over do it.
So the basic Heart Rate Monitor has a watch part and a strap that goes around your chest. the watch part is a pretty good sports watch but it also displays your heart rate.
Bear in my I'm simplifying things, but the idea is that we can determine our Maximum Heart Rate, and that we can benefit by running in certain zones. For instance, it is generally understood that running at 65% of the max is really good for weight loss. and that running at 85% is anaerobic.
Most HRM watches have a way of inputting your max HR and then can display what % you are at.
Good HRMs will till you at the end of your workout what your Max HR was, what your average was, and some will even show you how many minutes you were in each Zone.
Really Good HRMs will have interval timers, and alarms. so you could set it to beep after 5 minutes to let you know when your warm up is over. or you could set it to beep if your heart rate goes too high or too low.
The best HRMs will have some kind of computer interface and will automatically log your runs and graph them. Also computer interfaces are the best way to program complicated interval training programs.
When you start looking at the best HRMs you will find they have GPS built in. It turns out that having a GPS HRM watch gives you the most flexibility in programming runs and looking at computer graphs and displays.
Mostly you will find runners with GPS HRMs. In gym the GPS is useless. but the HRM is still very useful. on the treadmill and elliptical a HRM watch means you can let go of the handles and swing your arms. Plus you get a log of your activity.
I've tried Low end polars and timexes. They are both adequate. The Timex "Trainer" line is about $100. I like them because they have good interval timer features.
I ended up getting a Garmin 305. The 305 has a lot of features and doesn't cost as much as the newer models.
there is a blog called DC Rainmaker where a guy talks about HRMS and GPSs. He has compared a lot of the models and it's worth checking out.