Reading this thread with interest. To experiment with the concept, I bought one of those little $10 Bell handlebar bags a couple months ago. I really like having snacks, keys, wallet, iPhone, etc within reach, but I'm content to leave my tools and repair stuff in the under-seat bag.

I've gotta say that having one with a map pocket, that's a little more convenient to work while riding, and maybe a little bigger would be nice though.