Every mode, excepting time trials, are available in multiplayer mode. Multiplayer is still where the heart of this game is, and the experience of getting together. In the pre-release environment, the online multiplayer worked quite well, though its fun is hampered by the inability to throw screaming obscenities at the person who just slammed you with a volley of homing red shells. There are few thrills like loading up with a few other friends and playing through a set of courses - and those are well timed for a game experience, at about fifteen minutes each you're not going to take a surprisingly long time to complete a prix.
Customizable karts return from Mario Kart 7, with motorcycles and ATVs, hang gliders, parachutes, clouds, monster truck tires, and more. As you collect coins while playing, more and more variety of parts gets unlocked. It remains one of the most interesting parts of the modern series, and balancing your vehicle's various statistics to get the balance that pleases you the most is a more in-depth exercise than you might expect for the lightweight series. It's also wholly possible to simply build a bad kart from the available parts, which could be hard for some younger children, and some chassis are clearly in the game for the joke factor and little else. That aside, customization is how you'll get the majority of play out of this game. You can set ridiculous rules and restrictions, creating insane games of tiny mopeds and volleys of shells that will have you in stitches - or you can set up what are essentially mutliplayer time trials, building a set of rules for maximum competition based wholly on ability at driving.
The game also includes a post-to-Miiverse or Youtube feature called Mario Kart TV, where your recorded match highlights can be shared with others. You can safely dismiss it as a gimmick - about as goofy and useless as the ability to take selfies in Wind Waker HD was. The internet will have fun with it for a few weeks, and then it'll be over. Its worthlessness does nothing to hurt the game, but it's not helping either.
There are a few key changes that make Mario Kart 8 significantly better than MK7. Frankly, everything just seems to happen faster. I felt that you recovered from being hit faster, were brought back onto the course faster after falling off, and were affected less by hitting obstacles. It felt distinctly like the game was conspiring to be less punitive, more forgiving, and simply more fun than its predecessors. For example, whereas before the first-player-seeking blue shell was a mechanic that simply punished you for doing well, you can now counter that random pain with a music box item that wards off everything near you, destroying it. While there's no guarantee you'll get a music box, there's no guarantee that your opponent will ever get a blue shell.
Like much about the game, it balances frustrating randomness with exercises of supreme timing and skill. It balances the old, iterative parts with the shiny and new. It is very much Mario Kart: Take it or leave it.
Bottom Line: The core is the same as ever, but this is kart racing at its finest.
Recommendation: If you have a Wii U and even a modicum of interest, grab it.