@Markerrag -- some do it yourself types can do very well with installing flashing on a chimney that is flat because it is fairly easy to seal the gaps between the flashing and the chimney.
That all goes out the window with natural stone chimneys, however. Sealing the cracks around those is tough for even a professional.
Oh, and if your flashing does start to leak, you can do a quick fix by applying caulk to it so it will reseal. Still, that is not a permanent solution. If your flashing is leaking, you'll need to get it replaced sooner rather than later.