To give perspective, the Tour of Ireland had a hill at the end in Cork that I think was called "St. Patrick's Hill". It was a 23% grade and the riders had to climb it 3 times in a little ride around Cork that is similar to the ride around Paris on the Champs Elysees, except that is flat!!

http://www.youtube.com/watch?v=gP1pMW1aUFU

A bunch of the pros bailed on climbing that hill because they thought the danger and chance of injury just wasn't worth it.

For me, anything over 16% grade, unless it is really, really short hill, is a nice little walk with my bike !