I wear a Polar F6 (about $100). I use mine soley to base why I felt so good or so awful. The other night I attempted to mountain bike after a brief lay off. It has been about 2 weeks with no activity and my heartrate was staying high (it was also 100 degrees out). It gave me a clear picture that when I turned in early my body was needing it since my average HR was pretty high for me but I wasn't riding hard. Sometimes I have trouble listening to how I feel but when my HR won't come down I can be forced to see it in a number.