I got a HRM chest strap last month. I use it on all of my rides.

Last weekend I did a 176 mile charity ride. The first day was 100 miles, and I pushed myself too hard at the beginning. I was feeling a tired by the end, so the next day I decided to leave my Garmin on the HRM page, ignore my speed and stick to a comfortable heart rate. I settled on 145 BPM because it felt good.

My average speed was slower on day 2, but I spent much less time resting at break points. I finished the 76 miles earlier than most of my team and felt great compared to how I usually feel after these things.