The feature I like best is Average Heart Rate. It's handy for calculating Anerobic Threshold and calories burned.

I have a few HRMs and there are things I like and don't like about each. The Sports Instruments (late nineties model) has the functions I like but the buttons are difficult to engage. It's also bulkier than my other HRM.

My Vetta HRM/cycling computer is pretty good but it to is bulky and on rough terrain can fly off your bike.

My Polar is the best made but doesn't have Avg HRM function. The transmitter straps are also as expensive as some HRMs. Luckily my transmitter chest straps from my other HRMs work with the Polar ...

I'd like to have more functions and the ability to chart my entire workout but it's hard to justify shelling out that kind of jingo. It would be different if I had a training specific goal that was a priority to me.