My cars aren't heavily modded (yet) but I've done some upgrades. But the car I like the most is my Car-100. I'm putting it here because it's based on the bitcharg chasis (but longer).
The Car-100 for those who don't know is the SonyEricsson Bluetooth RC car that uses an SE phone as the TX. The pcb, body and chasis are different, but all the go bits are cross compatible with bitcharg. Bitcharg Rear axels, tyres, motors, front steering, suspension (the mounting lugs need shaving), batteries (but I think the Car-100 has 80mAh batts) are all useable.
The chasis is very similar but longer to accomodate the larger pcb with Bluetooth radio.
I've modded my Car-100 with the following:
Huge rear rims slipped over the original Car-100 axel (it uses orange ratio gears)
Guards enlarged to accomodate huge rears
Racing grill and Fuel rail decoration on engine bay (actually on/off switch - the engine is hidden under the pcb)
(pictures following)
and the following bit gear:

(fronts only)

(actually it's a 4.2, but I couldn't find the icon)