And this, to me, makes them almost useless. That said, there are optical HRMs that are accurate and aimed at athletes, but ... they're aimed at athletes. Fitbit/Jawbone/etc aren't.

http://www.mioglobal.com/Default.aspx http://4iiii.com/product/viva-mini/ (this is just a wrist strap, but they're about to release an all-in-one)