Here is an almost horrific example of a chimney that we swept recently:

They called us because they noticed that their chimney was on fire! After putting it out with an extinguisher, we got out there and they told us it hadn't been cleaned in awhile. But upon inspection, there was creosote almost an inch thick along the chimney! They were extremely lucky that the fireplace was built correctly, or else the damage would have been way worse that it was.