A cameo can play as a nice little Easter Egg that tucks itself into a movie. All of a sudden, a film treats you to a surprise appearance by one of your favorite actors. But then there's another person. Well, two eggs is still a pretty reasonable dish. But if the movie keeps piling them on, if can feel like you've eaten a whole basket full of them and boy does your stomach ache. There are some movies that get the recipe right and some who just overstuff themselves and spoil the entire meal.
Possibly in the "Ruined It" camp we have the trailer for They Came Together. There's a cavalcade of familiar faces that come in to support the Paul Rudd/Amy Poehler vehicle. We see people from Saturday Night Live and many Fox and NBC comedies, as well as Chris Meloni and Cobie Smulders. It got to the point that trying to guess who would be showing up next took precedent over following what the movie was about. That can be seen as a big detraction, and it remains to be seen how it might help or hurt the movie when it hits theaters.
Others in the "Ruined It" category: Saving Private Ryan. This really gritty movie opened with a scene visceral enough to make WWII vets leave theaters due to flashbacks. It was a realistic, immersive movie that opted for genuine emotion over hokey war movie stereotypes. Well, until a character portrayed by Ted Danson showed up and started shooting Nazis. "Wait... is that Sam Malone? What's next, Carla Tortelli smacking a Nazi with her serving tray?" A very poor casting choice in an otherwise stellar film.
When it comes to successes, though, we can look at movies like those in The Expendables franchise. People sit through the movies wondering what great '80s or '90s action hero is going to make an appearance. It's icing on the cake of the over-the-top feel of the films (which are riddled with scenes and people punching each other bloody). There's an almost satisfied sigh when a Chuck Norris shows up.
Additionally, a well-placed cameo can lift up the entire mood of the film: Sean Connery appearing at the end of Robin Hood, and all those superstars at the end of The Player (which was supposed to be about making a movie with a lot of no names).
Too many cameos and a movie risks actually turning people off. Without at least one recognizable person on the screen, you might never hook 'em in the first place. Like many things that come out of Hollywood, it's a guessing game as to what will or will not be successful.
Just see how full one's Easter Basket is at the end of the movie.