Very addictive. I've spent the entire weekend racing and have spent 15.00 for credits.
Mine runs on my Nexus 7 and the graphics and playability are awesome.
I'll "purchase" a game, but I will never buy credits... If you play "free to play", you never really own the game and they can change whatever you want. Plus, you'll probably spend more money in credits than the game is actually worth. Sucker.