I’m always interested in these demonstrations, having grown up with so many incorrect myths about plate armour proliferating, from knights being hoisted onto their horses with mini-cranes to them not being able to get up if they fell over. This demo is quite a good one, and well worth a look if you’re interested in this sort of thing.
They look a little clumsy at times, but are able to move pretty well, and it’s important to remember that actual knights would have spent years training in this gear, so would have been able to perform even more effectively than the guys in the video.
The video, I believe, was put together by people at the University of Geneva.