Personally I found that 95% of the climbs were rideable. The only issue was picking a good line and not having to stop if another rider came off in front of you, or there were too many walkers. There was one climb where I was having trouble keeping the front wheel down and eventually I lost too much momentum, gave in and got off to walk.
When I was riding, I was definitely passing plenty of people walking, so usually riding is faster than walking. If a hill is so steep that you are in bottom gear and barely making it, then you would probably be walking pretty slowly too.
It also depends on your muscles; when walking you are really stretching out and then compressing your calf muscles as you power up a hill. When riding you are using your quads and hip extensors more (which are much larger muscles).
I have to ride up a pretty steep hill every day after work, so I get a bit of practice at it, and will do my best to avoid walking any part of a course. I walked The Sledgehammer though.
Chris