I haven't used a different navigation system to compare it to, but it seems to work pretty good. The main reason I chose to get the Nav system is for the backup camera and touch screen. The backup camera makes parking much much easier, I'm surprised it isn't standard.
You, you're pretty much stuck with the pressing OK thing.
I live the nav. I've used many others before, and this is fairly accurate, as navs go. I like that even in tunnels (I'm in Pittsburgh baby) and around tall buildings etc, the nav works very well.
One feature the nav could use is to be able to find the closest landmark from your favs or the list they have onscreen - you know, for the nav to list the closest Italian restaurants to where you are at that point for example. My old nav (on my iPaq - remember those? - used to do that).
Someone had a qn about the voice; yup it can be deactivated, and the vol controlled quite easily.