I bought some POC VPD elbows and VPD 2.0 knees. They were tested on Saturday with 4 hour ride on my SS, so lots of pedaling in horrible mud.
The knee pads stayed put the entire time, they pinched a little but apart from that they were great. I wore them under some Howies ventile jeans to see how sweaty they got.
As for the elbow pads, they were also very good and really slim fitting. I wore them under 2 layers, a Gore windproof and my Endura Stealth, again they got sweaty but they didn’t irritate me one bit.
They’re expensive but they fit really well, are light, offer good protection and aren’t bulky at all.