I suppose that's a difficult question for someone other than you to answer.
It's really up to the individual user, I guess. A lot of people have brought up the thought that if you want to watch 5.1 movies, do so at home, where you can really enjoy them.
I'd imagine that the majority of the time, the average person will just listen to MP3's, which typically don't do 5.1. I've got a couple of DVD's that have 5.1 encoded audio on them, but compared to the number of stereo MP3's, it's fairly small. I'd imagine for me, 5.1 would be a waste of time and money, though, to be honest, I'll probably do it anyway.
Try looking into what it takes to get 5.1 to sound good in your car, then determine if it's worth it to you.