Aye, it can be cheap. The blue shell frustrates me, as it never benefits the person firing it. If you're in 9th, it doesn't really help you to take out the guy in 1st, so that the guy in 2nd can win. Seems a bit stupid to me. Red shells make much more sense.
Gotta say I agree about the bananas in first place, they're very useful for defence. Trail them behind you and you're pretty much safe from shells and the like. Can use the boxes and shells in this way too.
Gotta say, the golden mushroom has saved my bacon a few times, getting me from the middle of the pack to the front just before the race is over!