Hey,
Last year I drove from Poland (Krakow), and we did a tour of festivals - Hell's Pleasure, Headbangers and then onto Wacken, and back to Krakow. That was a good couple of thousand kilometres of road trip. Also did a driving tour of festivals in 2010, but didn't drive to Wacken (hired car afterwards for 2 other fests).
Definitely a cool experience, more so if you have some people in the car with you. Having a car at Wacken is cool, because you have somewhere to lock up your stuff, and you don't need to have everything in your tent. But with the good comes the bad - the car area fills up quickly, so you can be quite a long way away from the festival area. We were in camp R, which was a long walk, especially at night
We waited in the car traffic jam outside the village for much much longer than it took us to drive there from Hamburg.
If you have any other questions, let me know!