I have three:
http://www.ic.ucsb.edu/~gundam/Xmods.jpg
http://www.ic.ucsb.edu/~gundam/Xmods2.jpg
http://www.ic.ucsb.edu/~gundam/Xmods3.jpg
Doesn't look like much, but there's not much I want to do to the appearance of these cars. I'm waiting for some funds so I can by JGTC bodies for these babies. Endless Z, Esso Ultraflo Supra, Arta NSX... Oh man, I can't wait.
And they've seen a fair bit of action.
Upgrades: All three have metal bearings. The two Skylines have six cell mod, but the extra weight makes the upgrade seem pointless. I'm looking to get Li-ion upgrades, and stack the FET's to prepare for some of those extra hot motors. What I need in the immediate future are metal parts for the steering knuckles and tie rod. The other metal parts seem pretty pointless as well.