A very good kart racer that is fun all the way through!
The first couple of races you'll have to get used to the controls/handling, after having played it I find that the game controls very well.
Customization isn't really big, but you can modify all you racers to make your favorite character suit your playstyle.
The stages are where it's at in this game! Super imaginative, with the tracks changing with each lap driven. From a volcano spewing debris to destroy parts of the road, to an aquarium slowly filling up with water!

Is it as good as Mario Kart? Of course not! Only Mario Kart is as good as Mario Kart.
But Sonic & All-Stars Racing Transformed does enough things differently to be fun in it's own way!

Easily the best kart racer I've played. I mainly only played the single player components, but this game is still really fun from a single player perspective. It blows Sonic and Sega All Stars Racing out of the water.

The main appeal of this game to me is the sense of speed you get, and how you are constantly drifting, boosting, and doing tricks throughout each level. These mechanics make races thoroughly fun without using items to get an edge. It just feels so good to perfectly drift around bends and get the level 3 boost, it never got old for the 16 hours I played it.

The single player lacks any sort of story mode, but still has a world tour mode that's pretty fun, I love the way you get more stars for beating missions on higher difficulties which you can use to unlock characters and progress further. The Grand Prix mode is pretty typical of cart racers, with the exception of the fact that you start every race in the Grand Prix in last place for some reason, I would've preferred it to be like in Mario-Kart where you start each race in the spot you finished from the previous race. I didn't mess around too much with the time attack mode, but it was interesting in how it allows you to compete against increasingly faster ghosts, so it always encourages you to go for a better time which is cool.

I really like the hard difficulty in the single player, it has a great sense of speed and just the right amount of challenge, but there's also an expert difficulty which is just not fun with how absurdly difficult it can get. I honestly think that the AI specifically targets you over other AI racers here, and they have absurd precision with their projectile attacks, being able to regularly hit you even if there's absolutely no way they'd be able to see you. Expert difficulty feels like, even when you play perfectly, you need to be very lucky to win a race. I couldn't find anything that I did wrong when I lost races in this difficulty, I just got blasted by every AI racer on the map.

The map design is mostly great, I really like a lot of the maps and how they change over the course of the race, usually each lap is at least slightly different than eachother which is great. There are some small segments of maps where you will get slightly knocked upwards off the road, which causes you to lose all control until you hit the ground. This problem is most prevalent at the s-shaped bend at the end of the Billy Hatcher course, where, at the fastest speed-class unless you preemptively slow down, you will always get bumped upwards and get sent straight into a wall, which has caused me to lose dozens of races at the last second and is just annoying. Other than that, the levels are all great and my only other complaint is that there aren't very many maps and they pad out the courses by doubling it with mirrored courses. This padding is pretty lame, and I wish they would've just been more honest about their small amount of courses.

I played the PC version and I encountered an issue where the game would freeze occasionally. This freeze caused me to disconnect from an online match, but when playing offline I was able to pause and wasn't badly affected by this, but it is really annoying. There's a fix you can find on the website PCGamingWiki that fixed this problem for me, but I'm not sure if this fix would work on non-Nvidia graphics cards so beware.

Overall, this game is really fun, even from a single-player standpoint, this is extremely fun. I don't have many issues other than the expert difficulty being just evil, but you don't have to play on that difficulty if you don't want to, so it's not a big deal. The game does require more skill than a game like Mario-Kart to play and I really appreciate that. The feel of drifting and boosting definitely makes this game stand out for me.