I planned on sleeping in my car as much as possible on the trip from Seattle to New Hampshire, because I am broke. By 11:30 on the first day I was tired of driving, so I pulled into a rest stop outside of Bozeman, Montana. I had my blanket, a rolled towel for a pillow (yep, forgot a pillow), and a big heavy wrench to defend myself in case I was set upon by a crowd of wild vagrants.
I was expecting to be a bit sketched out by the whole sleeping-at-a-rest-stop thing, but it turned out great! After some experimenting about how to get comfortable in my tiny car (pro tip: move all the luggage to the front and sleep on the back seat with your legs curled up however will work), I was good to go. And I was not the only person sleeping there, which helped.
There were at least half a dozen other cars camping out at the rest stop, including a van with a young couple and their kid (they had a mattress, though, wussies), and a few other traveling college/grad-school aged guys. I fell asleep to the roar of I-90 and the glow of the young mom's booklight.
We all woke up at about the same time the next morning, when the sunlight became too bright to sleep through. Everybody got out of their cars to stretch and look around, then we all went into the bathroom to wash up, brush teeth, etc. Everybody was smiling at each other like we were some weird, temporary I-90 family. It was great.