Refurbished the Fixer-Upper car. Very cool looks and loads of Sims 2-esque interactions, but it is deadly slow... literally bicycle slow. "Work On Engine" interaction still available, so I'm not sure if further engine work will increase the speed. No love so far from my (Handy) Handiness 5 Sim.
Extremely good money-maker. Bought for $350, sells from inventory for over $35,000. Also great for leveling Handiness even when completed, as there seems to be no end to Handiness interactions ("Work On Engine" and "Wax Car"... though I would have expected the "Wax Car" option to level Martial Arts
).
Other Sims will interact with a not-yet-fixed Fixer-Upper: I left a fresh hulk in the sorority garage at University and the sisters all pitched in to finish it off while my Sim was in class (not sure if this has any bearing on the miserable speed, as the sisters all seemed to prefer bodywork over engine repair). Combined with the high sell price, this could be very profitable (maybe brokenly so).
Update: Apparently, Sims will interact with a finished car as well. My Sim was eating at the diner when a random Townie simply popped the bonnet and started hammering away. Fortunately, although the Go To options were unavailable on the car interactions, I was able to leave the lot normally, the car vanishing from under the ersatz mechanic.