I think everyone can agree his modern career (pretty much the last two decades) is just sad. But even before that, he was terribly hit-and-miss. Some early stuff was good, but after that anything that wasn't a "Vacation" movie was just terribly unfunny. His attempts at physical comedy were never amusing, and even less so when he still tries to do it now days. But this is a review of Fletch, so I'll just leave it at this: If you're like me and you still hadn't watched this movie after 25 years, don't bother. It's just Chevy doing his usual unfunny shtick.
Fletch is by far Chevy Chase's best film, mainly because of the control and compromise of director Michael Ritchie. Sure Chase gets to ad lib, but Ritchie holds the structure together in the long run and the film holds a silly noir feel that even though you could think of it as a parody of noir films, it really isn't. It's just the world that Fletch has fallen into.
Most '80's film age terribly, yet Fletch maintains a timeless feel even with references to the '80's Lakers and the soundtrack that is deeply trenched in the '80's. It's still a classic film and the best example of Chevy Chase at his best.