Per the game's title, your vehicle transforms during a race into a car, boat, or flying vehicle, depending on the track's terrain. Don't be fooled into thinking this is merely aesthetic; each transformation handles differently, with track sections on water feeling more like Wave Race than a traditional kart racer. The flying sections feel the most different, because the ability to move up and down freely, as opposed to just left or right, makes a big difference not only in where you go, but also in how you use items. You're less likely to hit enemies with a weapon when they have an additional axis on which to move around and dodge.
All of this is complemented by wonderfully designed and often dynamic tracks that change as you race. You may make three laps around one track, but it won't always feel like the same track each time, because you might be forced onto a different route through the environment your second or third time around. You might spend most of your first lap on land while your second lap is much more water-based. Paths and shortcuts change, making it a bit harder to know an entire track by heart. The transformations are scripted--there aren't moments when you're changing the landscape at will--but they keep tracks feeling fresh longer. All levels are based on different games from Sega history, including a Nights level that's particularly good at capturing the look and feel of the game that inspired it.
You aren't limited to the standard Grand Prix style of events, although that's still available and done well. The more interesting mode of progression is the World Tour, which is a map full of events to play and earn stars through. Events range from simple races to more focused tasks like drift challenges, and each event has different difficulty settings. The higher the difficulty setting, the more stars you earn. Most new events are unlocked as you finish the previous event on any difficulty, though some gates to new events (as well as unlockable characters and car mods) require you to obtain a certain number of stars to move forward. You won't be able to see the end of the game by playing on easy the entire time.
While perfectly fine alone, Sonic & All-Stars Racing Transformed is more fun with friends, who can join you in all game modes locally, even World Tour and Grand Prix. So you are never forced to play alone if you don't want to, provided you have at least one friend to share your couch. The cooperative options aren't available online, though, where 10 players are limited to simple Race and Battle modes.
Sonic & All-Stars Racing Transformed is a joy to play and look at. The icing on this delicious multilayered racing cake is that the game launches at a relatively budget price of $39.99, despite being a quality product that would warrant a higher price tag. It has occasional technical glitches, but it's a high-quality experience that deserves a spot on the winner's podium with some of the best kart racers you've ever seen.