My wife mentioned going to Cannes while we are in the French Riviera, however, Cannes is barely mentioned in the Rick Steve's book. Is it worth it? I only have 3-4 days in the French Riviera.
Not sure what you mean by worth it? There is not a lot there to see. The hill towns behind it are more interesting. It is probably worth an afternoon.
Just because its not mentioned in a RS book means absolutely nothing. I think Cannes in September is not a bad idea, main crowds are gone, weather likely still nice enough. Of course it is expensive and superficial but fun to see for a day or two anyways.
Having been there, I would avoid Cannes. As one responder says, there is not much of interest to see there. Do a google search for 'Cannes sights' and read about the area. You may find sights you want to see. Depends on your interests.