This one of those must have party games. It's great fun to play in either the co-op or versus modes. I'd really like to try the LAN modes, but I've never managed to get all of that together.
The ability to play with a team-mate adds more to the game than you'd expect. Being just a gunner may sound boring, but there are more things you can do, like help get a boost during a skid, and bash into people that come up along side you. In either role, you will be frantically trying to help win the race.
The various Grand Prix modes have the right mix of fun and challenge, and are most enjoyably beaten with a gunner at your side. Multi-player is great for hours of fun. 4 player All Cup is great for a long set of races. It's just all around good fun.