I've been deciding on going aftermarket as well. From other posts, a few people have successfully installed the Kenwood 6990 and similar models.
The biggest issue to me is that the screen will be a bit lower than the stock navigation and it doesn't have the simple stock look. If that doesn't bother you, then there won't be any problems.
For now, I use a Nexus 7 docked to my dash for my GPS/Movie/Pandora listening.
Montego Blue Metallic e92 6MT
|| BMW Performance SSK Kit || Charcoal Delete w/ K&N Drop-In || CDV Delete || Custom Clutch Stop || (3) Stage Manifold Swap || Stage 2 Eurocharged Tune (