No, no, we're not going hands-on with Mario Super Sluggers. I don't like baseball or Mario's wacky non-platforming adventures all that much. The fine fellows at Nintendo World Report are playing through an import copy, though. This is really good coverage, so the whole link is worth your time, but what interested me most was the in-depth description of what Challenge mode has to offer.
Challenge mode resembles a Mario platform game more than it does a baseball game. Starting the game as Mario, you need to travel around from stadium to stadium, trying to locate team players to add to your baseball team, eventually getting enough talent on it to defeat Bowser inwhat elsea game of baseball. As you explore the grounds around each park, solve puzzles, and clear special baseball challenges, new players will be added to your roster. Finding one of the team captains (the first player you would normally select in a regular exhibition match) will give your adventuring team a new ability to help unlock more of the world and find more characters and secrets.