The quality of Aux-in is rubbish. After changing the aux-in plug / cable, you may get the sound in other side but quality of sound is not good.
What I have found is that there is some sort of distortion when I use Aux in but its all good if I use CD player. I have tried multiple aux in cable but no avail.
I've bought O-scope recently and awaiting its delivery. With some experimentation, I will be able to confirm distortion with aux-in.
I am in the same boat as you are, however my preference is to get complete control of steering wheel buttons with ipod/iphone.
I have compared all these options;
BMW ipod kit (made by Alpine) Vs Dension kit Vs MoBridge
If you search their comparison, you will find the answer most relevant to your needs.
Since you have idrive, you can ask the part number of ipod kit compatible with your car from your dealer and get it from ebay (slightly used).
As Ctuna described that you will require to have your HU coded for it to accept. Do not go for old ipod kits which use aux in to send the sound signals to HU. Get the one which fits with MOST.
For my requirements, Dension worked out in terms of quality and best bet for its price. However some have reported MoBridge best. Again, only you can decide which one is best for you.
Remember, these is no point of spending thousands on your sound kit, if your primary source of input is a cause of distortion in your sound system. Use CDs/ MP3s or buy good ipod kit.
Considering the prices of these, I am now thinking of aftermarket Audiophile HU with built in DSP and ability to go completely active (at some stage). So no MS-8 / Bit one / Bit ten and with after market HU, I'll be able to acces ipod / iphone.
If my car ever goes to dealer, putting OEM HU is hardly 15 minutes job and I'll do that.