Watch the The Thumb & the Pick online guitar lesson by Henry Johnson from Jazz Expressions
Sometimes when people see me play, they see me play with my thumb or they see me play with a pick and ask me how I started with either one. When I was a teenager, I heard Wes Montgomery, and I was amazed that he was playing with his thumb. I started learning how to play guitar that way when I was 14. I've been playing that way since then, but I also use a pick sometimes.
My technique is a hybrid based on my experiences. Using the thumb has a mellowness to it that you can't get any other way, but the pick has a propulsion to it that you can't get either, so I go back and forth between them. However, in most scenarios, what I'm playing I can play with either one.