Oddly enough, this was the first Pink Panther film I ever saw. Thanks to its "flashback" scenes, I got interested in the PP series and eventually saw all the previous entries.
Although this film was clearly a bad idea from the beginning, the first half actually isn't too bad. I didn't realize, until it was pointed out, that all of Sellers' scenes were deleted (or reused) footage from previous films. I had thought (like some other people who've commented) that Sellers had actually filmed scenes for this movie but died before finishing; that's testimony that Edwards actually did a pretty good job of incorporating old footage into a new story.
Things start to go wrong when the Sellers footage runs out, at the moment Closeau's voice is dubbed in with the worst imitation imaginable. Soon almost all pretense of a plot is dropped. Now we get pointless interviews with past PP characters that comprise a tribute (often unbearably corny) to Closeau. He's discussed as if he's a movie character but the interviewees aren't; it all has nothing to do with how anyone would act if Closeau were real or any of the "plot" were actually happening. "Flashback" scenes are over-explained, and one in particular (involving Cato) comes so totally out of nowhere as to be confusing and disruptive. Finally, the scene with the "grape treaders" was totally unnecessary. (At least it was easily edited out for TV showings.)
Not everything's bad, though. Herbert Lom is the best thing about this movie; he holds it together as well as could be expected. Showing scenes from Closeau's early life was a good idea, even if it wasn't carried out well. The part with the dog guiding the old woman is original and funny. At the end plot points are left unresolved all over the place, but that's OK because we're being set up for a sequel.
One could say this movie shouldn't have been made. That's arguable, but it certainly could have been made better than it was.