Bob Wewer (on Chimney Flashings) discusses many issues in the trades and reading his posts will help to educate you of the potential problems that may arise with the quick air gunned-equipped roofers of today that carry the "Ice and Water" Underlayment to cover their shortcomings.
Bob Wewer has done roofing for over 35 years and only one time on one roof has he ever considered a chimney flashing good enough to retain and employ on a new installation. One time in Lakewood, NJ and the log gone craftsman actually signed his work (that was some 30 yrs. old at that time). His name was "Amel Otto." Bob checked and asked around the area and Amel was a legend leaving a legacy of fine work behind him. May Amel's name live in posterity as one of the good guys!
Not only neglected by roofers but even more by masons, chimneys are a sore spot for many that have had repair work done to them. See Bob Wewer's post on this.