Dan Harmon has a more diplomatic answer: Cage is Cage. He's neither good nor bad, but he is someone who can get the entire room talking about how good or bad he is. That's a special kind of talent.
It's important to note, however, that Nicolas Cage is someone for whom we all LINE UP to see do whatever. If I'm at a party at your house and I put Grown Ups 2 in the DVD player, nothing against that movie, but it's a party, people are going to eventually go into the kitchen so they can keep partying. If I put Ghost Rider 2 in, at some point, there's going to be a circle of people sitting cross legged around the TV. The party's going to become a Ghost Rider 2 party.
Here's a charitable look at Cage's career. The basic idea is he's a good actor who takes on bad roles because he has money problems. When he's in the right role, his acting skills shine. Roger Ebert explains why he's a good actor here.
And Cage. There are often lists of the great living male movie stars: De Niro, Nicholson and Pacino, usually. How often do you see the name of Nicolas Cage? He should always be up there. He's daring and fearless in his choice of roles, and unafraid to crawl out on a limb, saw it off and remain suspended in air. No one else can project inner trembling so effectively. Recall the opening scenes in "Leaving Las Vegas." See him in Scorsese's "Bringing Out the Dead." Think of the title character in "The Weather Man." Watch him melting down in "Adaptation." And then remember that he can also do a parachuting Elvis impersonator ("Honeymoon in Vegas"), a wild rock 'n' roller ("Wild at Heart"), a lovesick one-handed baker ("Moonstruck"), a straight-arrow Secret Service agent ("Guarding Tess") and on and on.
He alway seems so earnest. However improbable his character, he never winks at the audience. He is committed to the character with every atom and plays him as if he were him. His success in making Charlie Kaufman a neurotic mess and Donald Kaufman a carefree success story, in the same movie, comes largely from this gift. There are slight cosmetic differences between the two: Charlie usually needs a shave, Donald has a little more hair. But the real reason we can tell the twins apart, even when they're in the same trick shot, comes from within: Cage can tell them apart. He is always Charlie when he plays Charlie, always Donald when he plays Donald. Look and see.
If acting only required you to scream and yell at people, then Cage would be the best actor of our time. Too bad acting should require more range than that. It's true Cage had some amazing performances, but that doesn't excuse the fact that a lot of his movies have been objectively bad.
It’s now becoming tiring to see a wretched-looking Nicolas Cage playing a depressed bloke in every movie he’s in, and although his miserable face may be a reflection of his real life circumstances (which includes tax issues and debts) from the audience’s point of view, his films are beginning to congeal into a big gloomy mass; I seriously cannot tell one from the other. These latest filmic atrocities may simply be a string of pot boilers to a debt-ridden Cage, but that doesn’t explain the horrid shite he made during the early noughties since his financial troubles didn’t begin until several years ago. Regardless, the more drivel he appears in, the more his bankability as an Actor is waning, by the time he’s debt free he may never be able to recover from the back-to-back bullshit he’s made.