Interesting question and it seems to be discussed often. I have a 2011 E90 M3 and I put the 15/12 mm spacers on.
The "look" seems much improved, no vibrations or steering abnormalities. No uneven tire wear. However, the tar and dirt residue from the tires has increased. Not a problem for me, as the car gets cleaned ever one or two weeks.
Most people do not notice a steering feel change, but for me that was the biggest annoyance. I noticed a difference in the "stance feel" right away. Since the tires a little further apart, it felt like the center of gravity was off. Eventually I got used to it, but the feeling was definitely there at first. It did not impact the handling capabilities at all, it was just a feel.
My wife loves it and she uses the car to commute daily, so we kept them on.