What is the effective range of Magnet Shrooms?
The range of a Magnet-shroom is a 5x5 square, so you'll need at least two of them to cover all 6 lanes. They have a pretty long cooldown time before they can steal another metal item, so it's a good idea to have two if you're expecting a big group.
They still work against Digger Zombies even if the zombie is underground. He'll surface when he loses his pick.
It is theorized that Magnet-shrooms don't steal a Digger Zombie's helmet because they are made of plastic, but we may never be able to prove it.