When I started out running a few months ago, I decided to buy a HRM - got one quite cheaply at a local electronics shop sale for $25 ( this model usually around $50) . There are all kinds and this was about the cheapest of them all -- but it seems to work OK and does most of what I want. I have compared it's result against an Android phone App that measures heart rate and the results seem accurate (as in they both give the same result).
I initially got it because I am very conscious of my age and I wanted to make sure that I didn't blow a gasket!! But after using it for quite some time, I got confident that I wasn't overdoing it and started to leave it at home. However, as I have now started this programme - and also very aware that it will ultimately require me to run non-stop for 30 minutes, I have realised that I have become a bit too cocky and have been running too fast. So, I have gone back to the HRM to force myself to run inside the appropriate easy running zone. This has been hard - because my current cardio fitness is such that it is difficult to run slowly enough to keep the HR down to the correct number . But I have managed it and I am now thinking that I will also use it to slowly advance my HR during each week - with say a 5 BPM increase each of the 3 days of the week - and then run a 5K at Parkrun without it on Saturday at whatever HR/breathe rate that I feel capable of.
Interested to hear of other's use of these devices.