I worked for John back when computers were made of stone with little hampsters used to power them. He was one of the key people at OTI and really, really smart. I was one of the junior developers and really, really dumb.
John is one of the brains behind the IBMs (e.g. OTI) Smalltalk and Java VMs. He groks computers like no one I've ever met. His insight into how people program and how programs work is amazing. He's fun to be around as his mind works in weird and wonderful ways that make you "Wish I had said that".
Take for example his blog on "Visible ink"
Nice to find his blog.