Before I had any kind of skill in MK64, and mind you I barely ever played Super Mario Kart or much of any racing game, it was all a friend and I could do to get 2P going and once-and-for-all get the Gold Cup on 150cc Special Cup. ...then, after Mirror Mode was unlocked, we picked our jaws up off the floor, and beat it (or beat us, in the case of Mirror Toad's Turnpike).
Now? I beat all the Staff Ghosts, ace just about any CPU race. Can't do Quantum Drifts or SSMTs, but I really don't care.
As far as unlocks in this game go, I just want the Shooting Star because I've already got Funky Kong and Rosalina; if you've got them, you really don't need anything else, unless you have real driver chemistry with Baby Luigi/Daisy/Baby Daisy.
I should warn you, PB is a *BEAST* in MKWii.
And on top of my using "inferior" kart/character combos, I haven't even beat a Staff Ghost, yet. Got close on Mario Raceway N64, but that was before I unlocked stuff.