I have been testing the FR 55 vs various belts on and off since January - low intensity as well as short burst high intensity intervals.
I have to admit that I am very much surprised by the accuracy of the FR55. Yes, once in a while (but not very often) the sensor lags behind the first couple of minutes for some reasons, but from then on it is quite reliable and gives very similar readings as the belts I have been testing. Based on these impressions I do exclusively use the FR55 to broadcast HR to my Edge.I do not use belts for HR measuring anymore
Just as a positive feedback.