I've used the Polar RS400 for more than 5 seasons. It is a good piece of kit and when combined with the foot pod can help you train effectively.
I agree with ASM that a good rinse of the strap after each use prevents the build up of salt within the strip which impacts on the signal. The other thing which I noticed impacted on the performance of the unit was the battery in the strap unit and also the watch. Although they worked together, sometimes the signal would lock out or flat line at a particular heart rate. Once I swapped out the batteries in both, their performance was spot on.