Funny you mention Audi of all of all the other nav systems, only because the same magazines that pan iDrive love Audi MMI. And I would say that the Audi system is easy to use. I've only had a demo of iDrive, and I found it also easy and intuitive; the difference is the first and second tier menus, compared to the four corner-buttons. But the 4point main menu makes more sense than 8 buttons, so you don't have to look down.
Aside from all the options in the iMenu, the best part of iDrive compared to the Audi, from the demo I got at a dealership, is the ability to put a map on the Assistance Window. Audi's 'assistance window' is between the tach & speedo, which is good for navigation arrows or your trip computer, but it doesn't give as much information or a map.
Was: 2008 335xi sedan 6MT Black Sapphire Metallic
Is: 2013 Fiat 500 Abarth Nero (black metallic)
Wife's: 2013 Infiniti G37 S sedan 6MT, Black