There are a few BMW oriented tuners out there, like Alpina or AC Schnitzer, and their products are not just aesthetically pleasing, but also high quality.
Still, there are a lot of non-specialized tuners that have a lot to offer for the discerning customer.
A very good example would be Vilner, a Bulgarian company that specializes in interiors, and I have yet to see an ugly car to come out of their hands. There is also a similar company that hails from Poland, and also likes to fiddle with the inside, and they go by the name Carlex Design.
Carlex Design is not quite as well know as Vilner, but if we are to judge from the splendid work they did on this BMW 4 Series, we could say the future looks bright form them. They went with some high grade, natural black leather, nicely stitched together with red thread and even a few carbon fiber pieces here and there. Overall, a nice improvement.