I got a Road Tech 660 for Christmas, and purchased the navigation interface. The installation instructions were a little confusing on which connectors were to be used, but I was able to figure it out with my novice experience. If you don't intend to use the mp3 or XM part of the Zumo, then you don't have to plug it into the Aux jack. If you do plan to use those options, then you do have to plug it in.
I found the way it plugs in is not that bad. There is a connector in the fairing harness that you connect the aux cable to, and then run it out through the handlebar opening. The kit comes with a couple of clips that stick to the inner fairing to help hold the wire in place. I find it much neater than plugging in an mp3 into the aux jack.
Having the gps voice directions come through the stereo speakers is pretty nice, and is a lot easier to hear than it is through the internal speaker.