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.
Sakhir Orange M4