Multi-player wins the day in Sega Soccer Slam.
Now don't get me wrong, the game still has a solid single player experience, but I think you could probably wear it out during the course of a rental.
However, the multi-player really shines. If you'll always have some friends to share SSS with, you should definitely consider purchasing it. Even with just two players you can have a blast. The game features co-operative play that can be used in conjunction with any mode (as opposed to just in exhibition matches). I like co-operative play. A lot. Since you have three primary players per team (plus a goalie), you can have three players fight against the vile "expert" level computers. You can also set up for player matches that are 3 on 1 (plus two A.I. characters for the loner). I've been playing through Quest mode with my brother... he's using a small faster guy that is good at making shots, I'm using a meatier player that is more effective at punching people in the face.
Yes, you can punch people. This definitely isn't your usual Soccer game. It's arcade all the way. The derivative gameplay is peppered with enjoyable over-the-top additions such as spotlight shots, turbo boosts, and a power meter that can be used to give you limited boosts of strength for shooting and punching, or saved up to make one insane shot at the goal. The gameplay isn't necessarily very deep, but it's quite enjoyable and oriented towards teamwork. Get it for the multi-player.