Hi ssangjune. Well, it all depends on what you're looking for. You mentioned ones that sit on the dash and ones that come down from the roof, but have you thought about in-dash? I'm not sure if you want to go that route (taking the regular radio out, if that's even possible on the new E90), but if you're looking into aftermarket DVD players, I'd definitely recommend a Clarion DVD player, or any other brand of DVD players that can play VCDs, but clarion was always my favorite. I had on in my old car (which I sold to put towards the downpayment) but anyway, it was really great. It played DVD movies, and I actually was even able to download music videos from like Kazaa or edonkey, I'd edit the videos by replacing the audio with cd quality audio (audio taken from a cd and spliced into the video itself), and then burn the videos on a VCD and the clarion was actually able to play VCDs! It was amazing. I was the only guy in town to be riding around with music videos playing in my car at the time!
But that's only my two cents. Like I said, I don't even know if aftermarket radios are even an option for the E90 (I don't know how the whole factory system works), but that's definitely something to think about if you can in fact do that.