I have the "Xbox classic" with Forza.
A few weeks ago I hosted a Xbox night with a few fellow PCA-Boxster Autocrossers - LOTS of fun
I am planning to attend a track day at Laguna Seca this year with PCA and am practicing a lot for it on the Xbox. It's actually very interesting to take the official track guide from PCA and drive Laguna on the Xbox.
To see how times on a racing sim translate into real world times, have a look at this:
http://www.youtube.com/watch?v=L20IPTT_Wg4 where Jeremy Clarkson (TopGear) drives Laguna Seca first on a PS2 then he goes on the real track on a Honda NSX, quite interesting...