It's very tempting and very quick and nice solution, the only thing I can contribute is that I believe it's a better idea to have BT and Navi integrated in your multimedia system so that it mutes music/radio automatically once it realizes you're making/receiving a call or it's about to give you the navigation instruction. It's more expensive and harder to install, but I think it's worth it in the long run.

But if you don't listen to music/radio in your car, mirror would be the best way to go.