(if you have a mirror in bathroom you can see if really cranking some abdominal and pelvic muscles makes the blood vessels in your neck pop out - fun in the bathroom!)
Aikiweb has the best
discussions, I swear.
I don't think they're particularly difficult to learn (if someone manages to turn a little in a VNR, congratulations to them, they now have a forearm/biceps across their throat, which is a tad painful), and they do take effect quite quickly, but I don't think they're quick enough for multiple opponents.
The chap who had the stroke(s) was unlucky, but somehow I don't think a VNR was responsible ("my neck was forced into a compromising position", which the VNR does not do, it's actually strangely comfortable. It sounds more like one was applied and vigorous grappling ensued, which cause the torn artery), but still a good reminder to train safely and actually, you know, take advantage of modern medicine.