Tom Tom does supply the maps for iDrive. My portable Tom Tom fares no better than iDrive.
Personally, I use the MyBMW Remote app for iPhone which allows me to use Google search, and send my result directly to the iDrive for navigation. It also helps me find my car in a parking lot as well as being capable of unlocking doors, flashing lights, and blowing horn. Works beautifully for me (requires iDrive and BMW assist).