Polar F6 was my last one - it's great. The calorie counting feature is awesome. Unfortunately, I changed the batteries myself and after a year, it didn't seal properly and I ruined it with sweat.
My new one is a Polar ft4 and has self-changing batteries. It's not as "pretty" but has all the same features.
A tip - always spread a little water under the chest strap before you exercise. You get a lot better signal.