I tboned another vehicle once going about 45mph, without my seatbelt on. My head did impact the windshield, and that left me with a knot on my head, and a small cut. I also had a small cut on my knee from something under the steering wheel. I was otherwise unharmed.

I, of course, think it's imperative to wear your seatbelt, but our anecdotes don't really do much to tell that story.