This was supposed to be the GameCube's equivalent of Super Mario 64, but obviously things didn't pan out that way. Despite mixed reception, the game became commercially successful, found a devoted fanbase, and apparently warrants a sequel on 3DS (which isn't out yet as of this writing).
Mario and Luigi Mario was saved by Luigi in 2012's Luigi's Mansion, Dark Moon and in it's Prequel. They've been known as brothers since Luigi came out, and hopefully will stay that way. Brothers forever.