Need info on navigation system

I'm thinking about getting an '07/'08 335i, and am wondering if it's worthwhile getting one with the navigation system. As I understand it, it only works with a DVD--is that correct? If so, are updated DVDs available? I have two GPS units for cars that are easily updated from Garmin, so question the need for a built in system.
Also, does the BMW system work well? How is it for trip planning?
Thanks for any info.

You can get updated DVD for that year of car.
2010 has the hard drive version.