Polar heart rate monitor

Sepa

I have had my polar f11 for 4-5 years now. It had worked really well for me over the time I have had it until now. It does not seem to want to keep my heart rate on the watch and so is not giving me accurate calorie burns. How long is the usual "shelf life" for these watches? Should I look at investing in a newer model? I have seen the polar ft4 for £50. Is this any good?

Thanks in advanced