People don't seem to be using them currently.
Although they used to have a pretty good reputation before they were
taken over but German Maestro (I believe ) . My local stereo store says
they have gone down hill since then. If you look at posts that go back about
two to three years here you will see examples of people using them. The selection of speakers that fit is limited for these cars. There is a speaker list of those that do in the Audio upgrade start here sticky at the top of the page in the forum.
If you are new to this forum or BMW stereo you should read that upgrade start here sticky it will answer a lot of your questions.
http://www.e90post.com/forums/showthread.php?t=641323
I meant this one.
Morels , Rainbow , Jehnert and Earthquakes seem to be the poplular
speakers for the the stock positions and also Diamond audio.