I've also got a bike helmet and permission to borrow one for the summer. I was a little unsure about buying the bike helmet, because I hate spending more than $20 on anything, and the helmet was $40, but then I looked at it in two ways: A) this is my brain we're talking about and B) It's still less than a tank of gas.
I rather like my brain. And I hate buying gas, so I got the helmet. It's a decent one, too, and I've no regrets.
I do have a misgiving, though, and that is the fact that we live on the top of a large hill. Most of the time I do not think about this, but, while it makes going to anywhere surprisingly easy, it means I have difficulties getting back. Just yesterday a mere ride around the block necessitated a 20-minute nap for recovery. I managed to avoid the nap today, but my legs...let's just say they are not used to the exercise. But hopefully in a few weeks going around the block won't kill me, and maybe after that I'll be able to ride to work--which is at the farthest, lowest other end of this hill. So really, my hope is that eventually I'll be able to ride from work.