I bought it - so far it's great. If you have a GT3 save, you can transfer A & B licenses and 100,000 Cr onto your new GT4 save.
I've tried a couple cars to start out with (bought it, tuned it, deleted it, tried again with a different car), and the HPA R32 has been the best so far - the thing eats up the beginner competition like it's standing still.