I have creative zen. First one I bought broke down and had to return it for the same thing. The second ones software didn't work and I couldn't return it. But generally speaking it was a good product when everything was working. Although personally, I would go with an ipod for all it's versatility. Especially since some ipod models (depending on the car as well) can be directly linked to your car. http://ipodincar.net/ So, yes...ipod.
I prefer Creative myself...I have a Muvo that works great with the Creative sound card in my computer...as far as working in a car, my tahoe has line in, so it hooks from the headphones jack to the line in..I love it.
iPod hands down. I've had an iPod for 3 years now and I won't ever be switching to another mp3 player. It's an amazing product and I wouldn't ever recommend to anyone to get anything OTHER than an iPod.