I have a Pioneer DEH-80PRS in my E36 M3, and although the gloss black looks a bit out of place, at night it looks great because of the color matching. The sound quality is also great, plus it has bluetooth, etc.
I think what would bug me most, as was already mentioned, is the surrounding plastic. The head unit in the E36 is the perfect size, so you don't need any plastic around it.
Here's a shot at night of it in my E36:
And here's a shot of one in the light. Again, the polished black is a bit out of place but not too terrible:
Another shot in a different car:
Also, since the navigation display controls are on the stock head unit, how would you get around that? Or do you not have navigation?
-Rob