I gave up on the standard because at its tightest it wasn't tight enough - I suspect it was also too rigid so as I moved, the sensors weren't maintaining contact. The premium is much more comfortable and appears to be getting more sensible readings, but every so often still loses the plot - possibly if I'm not hot enough for the contacts to be sweaty enough? Or I'm wearing the "wrong" fabric?
It was worth it on the basis that before I got it, the HRM function was completely useless - at least I'm getting some sort of results, now. Today's 40 min run gave v consistent reading - suspect it was accurate and the fact that it was on the high side is a design flaw inherent to me, rather than the Garmin!