WELL, I have the 120gig Zune, so EVO can't replace that yet cuz right now you can only go as high as 32GIG micro chip...
However, I don't have 120GIG of stuff on my Zune and I did load a bunch of songs on my EVO, then got AudioPlayer with EQ and used my dual male cord to go from my EVO to my AUX in on my truck Stereo, just like I do with my Zune, and it works GREAT!!!!
When ever I use my EVO for Navigation or music I always plug it in to the Mobile charger anyway, so that's not an issue either.
I go to the gym with my Zune stuck in an arm band and since Zune gets Radio too, and the battery lasts longer, then I'll prob never stop using the Zune for that.