I must admit that i did go after market with my old 328 because i never really liked the standard alloys and i wanted more rubber on the road. The standard units looked to small for the body IMO. So i opted for a set of 18in 288/F40 style split rims with wider rubber. But yet i liked the standard rims on my Testarossa and would not have dreamed of putting on aftermarket ones because it looked right.
Also on my 355 spider i think the wheels are great and would not change them for aftermarket rims due to again IMO there are no wheels that are better than the standard units out there. The same goes with the 360, it looks right IMO.
So i guess its all in the individual eye

Oh and i am posting pictures of my past and pressent cars to add a little colour to the thread.