I only upgraded because I was thinking about an 800 for ages, then my Edge 500 broke and I didn't want to spend up to 10 days without being able to capture my power data while they fixed it. The 810 released about the same time, so I thought, just do it. Turns out all my power data is underestimating now anyway because of bugs with the 810, which is a major annoyance especially as we are now 4 weeks out from my racing season kicking off, this may explain why I am so bitter about the 810! But it is not the only annoying bug, there are many, most I could shrug off, but not being able to capture robust power data is an absolute failure, it is a basic data recording function, just like capturing data from a cadence/speed sensor, it angers me greatly (the total investment on the power meter and the Garmin is not trivial - think along the lines of a fairly high spec carbon road bike).
Turns out that it underestimates on average 7-8W average and normalised power and is fairly consistant in doing so, so I can use the data to train, BUT I can not compare my data collected with the 810 to the data with the 500, which means I am not able to quantify my progress, it also affects my TSS and IF calculations and messes with the performance manager chart in WKO+.