I just completed the Grand Prix's tonight, even the mirror modes. Unlocked some really cool karts. Cant really be assed with Missions.
Online is a blast, but its a bit of a lucky dip who wins. Not that much skill is needed and I find the best tactic is to stay back till the 2nd lap to get the better weopon then keep it till near the end of the race. keeping a lightning is very effective. One weopon i hate is the oil that covers your screen, it really pisses me off. I like the bullet weopon, but apart from that theres no new weopons.