Greg is an all around stand up guy. I personally like his presentation style and even though I've been hacking on the kernel for a while now at my current job, I never realized some of the things he presented. The checkpatch script is great, except that a lot of companies still ignore it. Freescale's BSP patches are extremely poor by checkpatch's standards, and that's what I spend most of my days fixing
Greg Kroah-Hartman is a great speaker & teacher. If you'd like to jump into Linux USB programming, his presentation on that is golden: http://www.kroah.com/linux/