I have a Polar M51. I got it for Xmas last year and I love it.
Ok, a little about my habits to give you an idea. I ride a fair amount, but not inordinate amounts of miles. By the end of this year, I will likely have gone about 2400 miles for the year. When it's warm, I'll go 5-6 days a week (assuming no plans) with one long 30-70 mile ride on the weekend - or an event. I ride for fun and to lose weight (which I have succeeded in this year: 40+ lbs). I do not ride to attempt to break a 6 hour century. At least not now. This is the end of only my second serious road riding season.
Having said that...
I use the following functions:
-Exercise time (duh)
-Time in zone
-OwnZone (but I set it myself)
-Total calories burned
-Accumulated calories
-Accumulated exercise time
-Average HR
-Actual watch features, like alarm
* Note that the 2 accumlated features are a good way to see how many calories you burn and how much time you exercise in a given time period. I reset mine every week.
I do not use these:
-Fitness test (forgot it even existed)
-OwnZone: self-calibrating. This is supposed to be a feature where you slowly warm up and it tells you where you should exercise that day. This was not remotely effective for me. I set my own zones before I ride every day.
-Percentage of fat burned.
-2 people can use the HRM
I wish I had these features:
-Maybe some sort of % of max HR number. But I don't think I'd really use this very often.
I do not care that I don't have these features:
-Cadence
-Compuer download. I think sometimes we all spend too much time in front of the computer and not enough riding. Just MHO.
-Lap stuff
-Time above/below zone. My first 1-2 minutes is below my lowest zone and after that it's all over. After a bit I stopped caring about over/under.
Ok, to address RonH's complaints...
1. I also once stopped the HRM in mid-exercise. Then I learned what not to do. I would assume that every HRM has a way to turn it off and I found it early on - in mid exercise.
2. The chest transmitter cannot be changed by the user. I have only had mine for 1 year and it is supposed to last 2500 hours. By my estimation, that's going to last me roughly 9 years if I continue at the miles/year pace I'm at now. Actually, I also use it for jogging and hiking, so I really don't expect it to last much more than 5 years.
3. As far as looks, I'm a pretty non-descript guy, so I prefer the plain black of the Polar. I took a look at the SportsInstruments that Ron linked, and I would have gone with the Polar on looks alone. But....well, that's obviously subjective.
4. I agree with Ron about the complications of many HRMs. They're just not necessary. If you made me, all I would really need is upper/lower bounds and total exercise time. I like the calories burned too. Those 3 features are all I really need. I think.
Hope this all helps.