Only the Road Tech versions of the Zumo work with the NIM to override audio and pass voice prompts. The NIM on my '12 CUSE7 works just fine and I can listen to XM, iPod, FM, etc., and still get the voice prompts. My NIM works great.
But when I use the RT 660 on my '07 which does not have the NIM, then as was said I must connect it to the Aux port on the radio and keep the radio in Aux mode in order to hear the voice prompts. So no XM, FM or any other audio input.
And I do not think this has changed - the RT Zumos come with a 2 year warranty, the non-RT versions from Garmin have a 1 year warranty.
And you can get the RT version from a 20% off dealer and that may be less than the non-RT version can be purchased.