I have a forearm sensor, and a chest one from Tickr. I find they're both very accurate, but they sometimes aren't getting the signal. Like, yesterday, it was stuck displaying 116 bpm for an extended period as I was doing a Tabata-timed HIIT with squats, pushups, and burpees. Once it 're-aligned', it shot up to the 140s.
Otherwise, I'm fairly pleased with them. I used to use the chest exclusively, but I found the batteries didn't last long. The forearm sensor is rechargeable.