I use Sigma Onyx Fit. I wear it all the time on my ride (except the casual ones).

I record the data on my log for every ride so that I get some idea about the distribution of my heart rate over three zones. I like to check the data, so just seeing how much calorie burnt is motivating to me
It's useful because you don't need to stop and count the heart rate. It also helpful in training.

If you can only afford the basic one (ie no three-zone setting etc), then don't bother buying one and just use your sense (perception of effort).