If you want pinpoint accuracy, get a chest strap. If you're just after more colour in your workout, and aren't interested in spending your sessions at specific bpms, a wrist-based monitor will do. Strapping on a good heart rate monitor is an easy way to supercharge your running or training. It not only makes your stats more accurate, but it also enables you to start heart rate training in specific zones, which can make your regime more efficient.
When it comes to finding a running watch I want something that delivers as much useful and accurate information for as little faff as possible. To that end, the Polar M200 answers my call: it works independently of my smartphone, it has GPS built in, it doesn't require charging after every run, and the interface is relatively easy to navigate.