I've tried the 150cc Grand Prix on Mario Kart several times now, and it always ends in me throwing my Classic Controller, more often than not rebounding on the cable attached to the Wii Remote that I'm sitting on, returning careening into my balls. Ouch.
Seriously, 150cc is hard and not fun. The CPUs always gun for you, and the number of items flying is silly. I was hit, not a dozen yards from the finish line on the new Mario Circuit, by a red shell, blue shell, POW block, and lightning bolt. Luckily I was so far ahead I finished first, but that's not always the case.
On the other hand, playing online is still a lot of fun. I played maybe a half-dozen matches and never finished lower than 4th place. I even got second on Rainbow Road! Human players can make the same mistakes I do, and aren't all gunning for me. Unfortunately the only way I know how to unlock all the characters is to do the Grand Prix competitions. Maybe I can get Greg to do them for me.