I've been using ArchiCad for about 10 years now (pretty excited by Version 10) and the workflow to C4D is great. Switching back to modelling changes from ArchiCad is pretty seemless as long as you take care of you materials properly in ArchiCad (which is necessary for good documentation anyway). I find it much easier to build the main model in ArchiCad due to it's great library of available parts and literally export straight from my elevation layer combination. All the components for 'real' images are best left for C4D (furniture, trees etc).
The one problem, as mentioned earlier, is the rendering. You certainly need advanced render for any decent results and the GI is pretty slow (especially for any large images) and awkward compared to third party render's that are available. I suppose it depends on the level of realism your after, as the scanline can be OK in some cases. The good news is Final Render S2, Maxwell and VRay (in the pipeline), are all available as plugins. I haven't made my choose yet, I was about to jump to Maxwell, then FR2, then Maxwell, then FR2 till I heard about Vray being ported soonish, so I'll have to make a decision soon.
Good luck in your decision, maybe see if the ArchiCad>Max combination can be updated from the original model. I can't tell you the number of times in the old days (V7) I set up the model perfectly in ArchiCad, exported it as a 3ds file, assigned materials etc and then the %*$#^% Architect (sometimes me
) decides to change a window layout. Nowadays (with the ArchiCad export plugin) I just update the ArchiCad model, export as a native C4D file and it updates the model in C4D. All the textures are retained. This way the ArchiCad/documents are updated AND it's almost instantaneously updates the C4D model to be re-rendered.
Cheers