It's an awesome experience with the feel of a "Chaos Draft" that uses multiple Magic Draft Boosters from different releases but bundles the cards and variety you'd experience into one amazing package. The set itself contains a whopping 1,670 reprints from across the game's history (see below for the full list), plus 121 copies of something new and surprising—R&D Playtest cards you won't find anywhere outside of Mystery Booster: Convention Edition. There are so many cards packed into the possibilities, you could fill an entire eight-person draft and not see the same card twice.
Each Mystery Booster: Convention Edition booster pack contains fifteen cards: fourteen from the gigantically awesome main set, and one from that "something new and surprising" group of R&D Playtest cards. You can check out these unique cards on Gatherer and revisit the Mystery Booster Release Notes and Gavin Verhey's article about the development of Mystery Booster to learn even more. We also have a full list of R&D Playtest card names and the artists if you're curious about the unique visual illustrations these cards feature.