Permanent smile
On the 750-mile trip back from the dealership to my home in my "new-to-me" 2001 Insight CVT, I also had such a smile that my cheeks hurt. You keep a massive grin going for 13 hours, you get that way.
The other posters are right... eventually that smile settles down to somewhere between a self-satisfied grin and an outright smirk. This seems to depend on one's mood whether one is being passed and thinking "yeah, you passed me, but I'm getting three times your gas mileage", or if one is doing the passing saying "Who's your daddy on the 1200-foot climb now?"
It's a ton of fun. I just got back from another two-hour drive, averaging 60.1 mpg on my CVT. Joy to drive, and I keep trying for a higher score on the dashboard video game!
I have a friend with a miles-per-gallon monitor on the dash of his SUV. He says "My miles per gallon display on my truck reminds me of a game of golf. It seems my vehicle thinks a lower score is better." He was very impressed with my Insight, needless to say
I printed up a little sticker and stuck it just above the H on my steering wheel. It has two words:
BEAT EPA
As long as I'm beating the 57-MPG EPA rating on my car for each tank of gas, I'm pretty well satisfied

Congrats on your purchase, it's just a TOTALLY FREAKING AWESOME little car! I sure love mine; I'm just terrified daily that someone is going to hit me while staring at my car and zipping past at 20 MPH over the speed limit...