I say this not because I fear it will land on someone's head (unlikely) or because its fuel is a poison (though it is), but because smaller pieces pose a lesser environmental danger both down here on terra firma and up in space.
One purported motive for getting the satellite out of the sky was to neutralize the danger posed by its 1,000-pound fuel tank brimming with toxic hydrazine. Certainly hydrazine is not something you'd want to stir into your coffee. The EPA describes its effects this way: "Exposure to high levels of hydrazine may include irritation of the eyes, nose, and throat, dizziness, headache, nausea, pulmonary edema, seizures, and coma in humans."
A condition potentially worse than throat irritation or coma is something called Kessler syndrome. It's only a theory, but Kessler syndrome refers to a situation created when objects in orbit are struck by other objects. They fracture, goes the theory, and create further (smaller) objects. All this orbiting debris spins around and around, posing a threat all the while to passing satellites and spacecraft.
Once objects reach lower orbits they eventually fall into Earth's atmosphere and usually break up on re-entry -- if the pieces are small enough.
How long does debris stay up there? Depends on its position in the sky. Debris from a weather satellite destroyed in high orbit by China last year will exist for "thousands of years," says MIT's Geoffrey Forden. "It is likely that shooting down the [U.S.] spy sat would [create debris with] much lower perigees (roughly at most 250 km as opposed to 850 km for the China's test.) That means that they would decay much, much faster."
Thousands of years? And the possibility that chunks of space junk will take out other -- working -- satellites and manned craft in the meantime? I'll take smithereens any day.
If you're really curious about how to take down a satellite, check out this illustrated guide: 8 Ways To Blow Up A Satellite.