I have a friend of mine who discontinued reading "The Complete Peanuts" series with the 1971-1972 volume primarily due to the introduction of Rerun Van Pelt. I plan to get all the volumes of this series, but realize that once the series gets into the 90s especially, the material gets a bit shaky (content-wise and literally), as Schulz did not have as much control with his pen line and also seemed to be very content in his life so the abrasive humor of earlier strips was replaced by many, MANY cute strips.
The 1975-1976 volume is still good and for my friend, there's only three strips in the entire book that have Rerun in them, so it's still quality stuff. I will say that the best years of "Peanuts" are probably 1955-1975, but that's the BEST. The strip was good before 1955, and for quite a long time after.
Like I said, the series will get a little trying once it gets into the 1990s, but I am going to brave it out until the bitter end as even a bad "Peanuts" strip is sometimes better than most of what's coming out today in comic strips.