VERY underrated. So what if it has a screwed up storyline? It's still an awesome game!
The graphics are impressive and the graphical effects in the races often give a very distinct sense of speed, which is a good thing because that's what Sonic's all about. Plus the textures and shading of everything is really good too.
The best thing about this game is the music. Each racing course has its own music, and, although there isn't that much, the music is all amazing and always makes the races even more epic.
Although a lot of people say the controls are bad, they're not bad at all. I always play holding the wii remote horizontally, though, instead of vertically. So I guess the people who complain about the controls play the other way(even though you can also play with the gamecube controller).
It took me less than 30 minutes to teach my older brother how to play, and how to do everything you can do in a race (a LOT of stuff). That's proof enough that the controls AREN'T hard to learn.
Also, in addition to its other pros, you can play as pretty much any Sonic character, even Silver the Hedgehog and Blaze the Cat, and even some non-Sonic characters, and each racer has their own stats and racing capabilities. I almost always play as Jet the Hawk, so I've gotten used to his controls and usually win 1st place in every race.
One of the only cons besides the storyline is that the Story mode isn't even a story mode, really. It's just a bunch of races with a random character. Another thing is that 2P battle mode isn't fun, not at all. You can choose from firing missiles at each other or struggling with gravity power to try to shoot a ball into a goal to score a point. Though it may sound sort of fun, it's not. It's just monotonous. Also, the "gimmicks" you can get from the item boxes during a race usually don't contain anything helpful at all. The only thing you can get that you can actually use is a bomb that rolls towards the nearest racer and blows up.
But other than that, this is really a good game. Don't believe all those idiots who complain that this isn't a fun game. It's a high-speed racing experience, perfect for a Sonic game.