I used my old wrist HRM by strapping it to a 2 inch piece of pipe insulation on the bars. It was great for pacing on long climbs. I could stay in the 140-145 range for hours, maybe 5 minutes at a time above 150, and a minute at 155+. It had settable alarms for too high or too low, but I didn't use them. I never really used the average for the ride, and it didn't track the maximum heart rate either. So a simple monitor was fine.
Edit--exactly like TomD77 said..
(Now, I got a Garmin 705 for following uploaded mapped routes. It also records the speed, heart rate, elevation and cadence. I do like reviewing ride data on my PC when I get home. )