SP will be hit or miss with some... MP is definitely where it's at if you get 10 + (up to 20) racers.
But SP:
it's not very story driven although it has a linear plot of having a boss for each area of progression. There is a voice over guiding you through. Each area boss has a list of qualifications that you must meet in order to qualify to challenge that boss to race a 1 on 1 to advance to next level. There are different fan levels you can attain for each event so although you may pass a boss, you can return to get a higher score in each race and go for completion.
yea there are several different types of cars (Street, off-road, drag etc) that as you unlock you can choose to use. there are 55 licensed cars in the game and they absolutely increase with faster cars as you progress... there are also Mods that you can choose form and add up to 3 for your car which give different abilities (higher shields, ramming power, evasion etc). so there is a lot of customizing of th game experience not really of the cars.
This game is played best online or as LAN or split screen maybe? party game or online MP.
the best thing SP did for me is it is teaching me how to play because there is actually a bit of depth to the game especially when online against some good players. I have yet to win an online race and only came in 3rd once.
ther game is chock full of little details, options and polish however.
although it lacks a custom race option for SP out of the box. I'm hoping it unlocks after completing career mode though. There is a custom race maker online or split screen (VERY ROBUST) but you need two people minimum. I would have preferred a custom race maker with AI too.