I have no direct personal experience with him, but I have read his books "Full Catastrophe Living" and "Wherever You Go, There You Are". Zinn has been pretty instrumental in the recent 'medical meditation' movement, and as such he is able to put mindfulness meditation into an easily understood language for Westerners. As well, he draws upon cognitive science to explain how the brain changes due to meditation. The meditation comes from the Buddhist tradition, but is non-sectarian, and not really 'religious practice' in any way.
As for your past skepticism, I think as far as mindfulness meditation is concerned it's pretty hard to deny the positive benefits in light of recent research. The works by Zinn will help explain all that better than I can.
I will say that for me mindfulness meditation has helped immensely. On the m.a. side of things, it has certainly helped me focus, have less tension in my body (so I'm more fluid than I was before), as well as having MUCH better breath control (and with that more stamina). On the non-m.a. side of things, it has helped me quit smoking, improve my focus for school and life in general and generally be a calmer person. Sometimes I practice for an hour or two a day, but usual it's only ten minutes in the morning and 5 or 10 before bed.
"Seek not to follow in the footsteps of the men of old; seek what they sought."