Eh.
Unlike many other series, Mario Kart is the most successful one to test my patience. I yell at the TV screen (or, in the case of MKDS, my DS) when playing a Mario Kart game much more than any other game I play. And with the now frequent occurences of lightning bolts and blue shells and uselessness of fake item boxes as shields for red shells, I may very well yell until my voice becomes hoarse.
You can beat the computer/staff ghosts in time, yes. Doesn't really make you good, though.
Can I beat others? I'm still figuring that out, because since before MKDS, I usually played alone. MK64 was the only MK game I played with someone else, my brother.
Online I've been able to defeat a few other players, strangers and friends (note online I will ALWAYS use the Wiimote + Wheel configuration, provided the batteries stay alive).
Currently, I only unlocked the Mirror mode earlier tonight (I'm going at my own pace, to enjoy the game rather than blow through it in a week) as well as Daisy. (Still not caring to put spoiler tags over secret characters, eh, BP?)
I've still got Mii B, Funky Kong, Baby Player 2, and Dry Bowser to unlock, and I believe the former two will take awhile, as well as a few karts and bikes (I'm not sure how many there are supposed to be for each class, 5 of each for all 3 weights? Or 6?)