I think they are a great help to let you learn how your body is reacting to your exercise. Its interesting to me how some days I feel like I'm going hard, but my HRM tells me "no, not really there puss.." and other days I can push myself much harder without knowing it until my HRM shows me.
It helps you moderate on recovery days, or times when you want to stay in a certain HR zone.
Its a tool, and like any tool, it's only worth it's weight if you use it!
I have found that if you search on the auction sites and on line, you can get the older Garmin packages (small watch and HR chest strap) for $40-60 (usually factory refurbished). That's where I got my first one and it is still running great years later.