1) Yes, the tracks are loads of fun. The game is fun to play, period. It has the satisfying slingshot-style drifting of MK8D (not exactly the same, mind you), and the game runs at a speed similar to the 200cc mode in that game, but here the tracks are actually designed for that speed. There are destructible elements, branching paths, and all manner of points where you're sent flying. There are about nine core tracks that are then remixed with different starting points and directions, time of day, weather, climate (i.e. snow instead of desert), mirrored layouts, altered side paths and collectibles, and unique setpiece spectacle. It's great.
2) The vehicles have different handling, but what's especially fun is upgrading them, which is largely cosmetic but dramatically changes their appearance (you can toggle upgrades on/off if you want to revert to earlier styles). It's weirdly addictive, like leveling up a Pokémon to see what new form it will take. There are 23 vehicles, most of which you unlock by completing tours and finding the 87 hidden keys, and then you use in-game cash (or keys) to buy them, at which point you unlock upgrades by winning races and leveling up (you will usually need 20 first-place finishes to max out a vehicle), and you then buy those upgrades once they're unlocked, so you will want to target the stacks of cash found on the tracks as well. Upgrades include neon, decals, body kits and engines; a huge selection of paint colors is available for each ride by default.