I've been using a HRM consistently for about a year now. Tracking all information in excel. It's a great motivation for me to track all training data, showing gaps when I'm not doing enough, but also showing me the improvements in time and heart rate I've made this far on trails I often take.
Training wise, I do think it's helped me, as it allowed me not to push myself too hard on certain days and harder on others. Especially on interval training days, it's been essential as before I'm sure I never pushed hard enough. In short: for me it's been a helpful tool.