I've been a Mario Go Kart fan since SNES...and I've picked Yoshi everytime.
On Wii it seems every time your near the finish line on the last lap either a red or blue shell smacks into you. heh. Annoying.
Of course there ARE ways of dodging both red and blue shells. With red if you have a shell on hand or banana you can simply use it to block the impact by holding the item behind you. (The red blocks do not work)
With the blue shells if you have a mushroom, wait until it's hovering above you then use the mushroom. If you time it right you'll dodge the blue shell.