Lots of intakes on the market for BMW's but really only a few good ones in my opinion. I use the Dinan CAI and stage 2 software upgrade. Big difference for sure. Away from the sound, which is awesome, the real noticable improvement has been the torque over the most of the rpm range. Remember the software programming has been changed too.
The other good one on the market is Active Autowerke in Florida...
http://www.activeautowerke.com/
They have a pretty good setup with alot of satisfied customers. Both chip reprogramming and CAI.
The other is from, I believe is UUC. Check Turner Motorsport....
http://www.turnermotorsport.com/
These guys, which I have spoken to this weekend at Mid Ohio for the Touring Car race, have a race proven program which supports their R&D for the products.
Does this mean one is better than the other, maybe, but these three are all pretty darn good, Dinan being the most expensive of the three.
I went with Dinan because BMW supported and approved, and of course estectics. You gotta like the looks of the Carbon fiber.
People mix and match CAI and chips all the time fro a tenth here and there. For me I like one product that has been designed together.
I hope this speach will help.