Like any series, the quality seems to fade over the life of the show. No difference here. The first six seasons were interesting and fun watching the two (Richard and Kate) spar with each other as they solved crimes. Unfortunately, someone decided that there would be a theme for each season in addition to the arc over each show/season of who killed Kate's mother.
The show lost it's steam after Rick disappeared after the wedding and the "heat" between the characters faded. The eighth season was the worst as the writers sped up to resolve all the issues knowing that the show might not have a ninth season. When Stana Katic's contract wasn't renewed, the show was doomed and they came up with numerous hours of drivel.
To tie up the show, they flashed a scene at the end which supposedly gives us a window on the main characters' future. Not convincing.