A HRM is only really properly useful if you're seriously training. Having said that, I use a Polar S720i as a cycle computer on my main bike as I find its other functions useful such as temperature, altitude and the like. A GPS system would do the same job for less cash though.
If you're cycling to lose weight then a HRM is useful though as (generally) your body burns fat best within a certain heart rate range and a HRM can help keep you within this 'fat burning' range. Having said that, maintaining this range is difficult when you're MTBing because of the variations in terrain. Better on a road bike.
I don't take training seriously so don't really need the Polar, but I like having lots of numbers to play with and comparing stats for different rides so if you're a bit of a numbers geek like me.. go buy one!
If you're cycling to lose weight then a HRM is useful though as (generally) your body burns fat best within a certain heart rate range and a HRM can help keep you within this 'fat burning' range. Having said that, maintaining this range is difficult when you're MTBing because of the variations in terrain. Better on a road bike.
I don't take training seriously so don't really need the Polar, but I like having lots of numbers to play with and comparing stats for different rides so if you're a bit of a numbers geek like me.. go buy one!