I found a HRM handy, as it let me work approx what I was burning in an average workout and how hard I need to/or not to push myself (like when walking the dog). So I found it was not needed for every workout, but handy to put on every now and then. I did not get mine till about half way through the season and I was already getting results.

There are times I wish I got a pedometer instead (contemplating FitBit), just so I can see what I am doing on a daily basis outside of workouts.