As far as the whole co-op thing goes, I actually liked being the backseat driver! At first glance you'd think that the only job was to toss items, but there's so much more strategy involved in that position. You actually had the ability to sway the kart left or right using L or R, and this could do anything from help them dodge a Green Shell/Banana to avoiding falling into a pit for P1 making a driving mistake. Swaying into a kart adjacent to you would cause you to punch them, which not only hit them but would steal their item if they were carrying once. The backseat position is actually pretty important in its own right and serves to complement or correct the playing style of the driver. I think the one thing that would've been nice was some kind of rear-view cam; since you're back there you should actually be able to see who's behind you and what's coming at you. Would be nice as you could in theory prepare for what's coming and try to intercept it. I do agree with PB's complaints about Double Dash as a whole, but I thought that the co-op mechanic was interesting and maybe could be revisited in a game that's more balanced as an optional mode of play.
I do think that Super Circuit is a very underrated game, though. It doesn't suffer from a lot of the balance problems that are discussed in the newer titles, and plays like a neat fusion of SMK and MK64. It did have a very interesting gimmick for its Rainbow Road, in that the majority of the walls were jump panels and could do anything from creating a shortcut to careening you off of the edge of the stage. A lot of the tracks and music were nice too, such as Snow Land, Cheese Land, Boo Lake, Sky Garden, and the Bowser tracks.