I really like the Zune HD but I am not sure it is for everyone and I am not sure it is the best thing going for just a straight music player. The interface is slick but, if you are just talking music, that probably doesn't matter as much. And, if your primary use is music for the car, aren't there other options out there that can integrate to a car stereo easier?
Personally, as much as I love it, if there wasn't a Zune Pass, I probably wouldn't even own a Zune HD...even though I do use the player for more than just music (podcasts and a little bit of internet).
The Zune HD is slick and fun to use but most of that enjoyment, for me anyway, is Zune Pass related...smartdj, marketplace on the tv, integrated marketplace on the player, channels, etc. Without that stuff, I could be using just about any decent mp3 player and be just fine.