I just upgraded from 2006 TL Navigator to 2008, now a 2 CD set. Obviously I find it useful if I bought a new edition to replace the previous one.
I like the convenience of searching an area for campgrounds on the computer, but in some situations the old book is easier too. The Navigator works good if you have a place name and want to see campgrounds within10, or 20 or 50 miles of that destination. It is a bit less handy if you want to scan ahead along a route and consider alternate destinations based on campground availability, i.e. you do not have a fixed destination in mind for the next stopover. I find the paper maps in the campground guide handier for that.