I've been having complaints about the Grand Prix's at Mario Kart Wii as well!... In the 150CC (And even a little in the 100CC) it's also a matter of luck which decides the outcome of a race! You can race as good as you want, everytime that i'm standing in 1st place for like 2-2.5 laps... I get hit with for example; a blue shell, a red shell, and then someone nocks me to the side, thus loosing like ~10 seconds and ending in 8th place... I'm trying to get a 2-star ranking in all grand prix levels but this annoying AI is really making it hard to get such a ranking in the higher CC-classes, however not impossible! It really doesn't come down to racing skills for your ranking, i believe my racing-skills are 'good enough' for a star ranking, only if you get hit on the wrong times by really overpowered stuff multiple times, i really can't help it... And then the game gives me a (for example) A or B rankings, bullcrap if you ask me! And that is what i REALLY HATE about Mario Kart Wii! This was, to a lesser extend, also in Mario Kart Double Dash!! However, the DS-version had a much 'fairer' AI.
Time Trials and online rankings, downloading ghosts from friends and trying to best eachother on tracks makes more then up for that though!
THE NETHERLANDS
