I have a Polar s720i and have had huge problems with interference from power lines. I have not yet called Polar so don't know what to tell you about its problems.

I recently bought a Ciclo 434. No HRM and no download feature but the things works really, really well. Seems to be accurate. Works consistently and the buttons are intuitive and easy to use on the bike. The screen is also easily read on the bike.

My LBS showed me a drawer full of Vetta parts which convinced me that that was not the way to go.