I have my own method, similar it seems to mrgraf it sounds like, I wrote a tutorial on it in the tutorial section here. ANyhow what I do is start with a part of the car, usually the door, but lately its been the front and I start modeling. I'll start in an area start extruding in a very low poly (boxy) mesh and extrude it , I'll then apply a turbosmooth modifier (not meshsmooth) 1 iteration and see how I like the progress. I'll continue on and extrude more and continually check my work in turbosmooth. I however have 3 methods to keep the mesh straight and not overly curvy which I talk about in my tutorial (shameless plug I know
) But I dont think its slow I can build a car pretty fast, I had my Aston Martin body built in like 3 days. HOWEVER my method is for low poly modeling (ie game cars and such) so when I say low poly I mean between 20-30k and up. Depending on the amount of detail you put in after my method.