I guess I'm still old school.
I got a mild concussion about a month ago and didn't bother going to the doctor about it because there's nothing they can do besides observe you for it, which is a waste of money for a mild concussion, IMO.
Between football, boxing, and other occasions where I've rattled my coconut, I've had like ten of them in my life and I know what a really bad one feels like and I know what a mild one feels like. When you've got a serious concussion, you know it. You're loopier than hell, your memory is completely ****ed, your head feels like it's splitting in half, and you are pretty much dumber than a spoon for a while.
When you have a mild one, you should know it, too. If you didn't have anything to drink, but you feel like you're buzzed (and not a good buzz), even borderline drunk, and you've got a killer headache, you probably have one.
Granted, for a head injury seeing a doctor is probably the smart thing to do, but I'm not really likely to go to a doctor over a mild concussion simply because I've had them enough times to know what to do. I definitely wouldn't go to an ER over one. A major concussion, I'd go to an ER for, especially if I lost consciousness. Of course, the two times I've done that I was eventually released a few hours later with nothing more than a whopper of a medical bill and headache to show for it.