In reading the review from the guy who ordered from Discovery Store, yeah, they have all the seasons. However, if you're ordering from Canada, they will KILL you on shipping. I was going to order a Mythbusters t-shirt for my son last Christmas, a $19.95 T-shirt BUT nearly $50 shipping to Canada! Forget it!
The show, however, is absolutely awesome, no matter what season you get. It's fun to get the old ones because they're so awkward around the camera. By the 7th season they're old pros, but because they're so used to being filmed, the hilarity comes naturally. The weird chemistry between Adam and Jamie really works. Great show, makes science fun and accessible.