I bought a 360 for work last summer and paired it with my old Motorola phone. The speaker quality is great - it was very easy to hear the speaker on the other end, even in my loud car. The mic quality, however, is only so-so. Frequently, I had to just kick the conversation back to the phone, because the other person could hardly hear what I was saying. Eventually, I just bought a bluetooth headset and used it in the car instead.
I haven't tried pairing my Nüvi with my iPhone, though. If I remember I'll give it a try tonight.