I have to agree to 1twojay, I'm almost at 50% and, as money is not a problem now, I'm playing "Buy That Used Car" to increase the number of machines available at the Arcade Mode. And, driving for fun, obviously, just for the pleasure.
Last weekend, I took my GT4 to my cousin's house to have some 2-player battles. And, boy, that was "da bomb". The Impreza vs. Lancer challenges at Tsukuba were amazing. But then, I found my new best car: the Suzuki GSRX/4 concept. That thing is extremely light and, in spite of its 1300cc Hayabusa sportbike engine, this little thing is faster in Tsukuba than any premium japanese sports car in stock form, even the almighty GT-R M-Spec Nür. Try it and you'll know what I mean.