I've been to all but last year's Overland Expo West, having instructed at the event each time.
If you're planning to take the trip, I'd suggest getting the experience package that gives you access to all the classes and driving instruction. The presenters and trainers have amazing amounts of experience, and many times have shaped off road driving to what it is today. (The Camel Trophy crew comes to mind, as well as others.)
You'll find a vast array of vendors, from the big auto manufacturers like Jeep and Land Rover, to outfitters such as Adventure Trailers, Sportsmobile and Equipt, along with thousands of enthusiasts who themselves have travelled the world. It's a great group of folks to hang around with for a few days!
-H-