I look fairly often, thanks to the habit instilled in me by working with Stephen James O'Meara too long. I suspect that average people don't look up at all unless there's something otherwise drawing their attention to the sky.
Rainbows get a lot of attention because they're so pretty.
Whenever a relatively low Sun behind you shines into heavy rain or mist in front of you, you'll see a rainbow. Many other atmospheric phenomena go largely unnoticed. Some, namely sundogs and Sun pillars, are more often spotted, especially when they appear low in the sky during morning or evening commutes from school and work. If you want a more rigorous model, you can't go wrong with the freeware program HALO by Les Cowley and Mike Schroeder. It can be found at their atmospheric halo home page .