They are perfectly harmless and very cool to watch, IMHO.
I recall years ago playing golf in Western PA when they hatched. The 1st hole was wide open in a meadow, but at the 2nd hole you went back into the woods to get to the tee box. I heard them even before we got into the woods. I thought it was some big pump with bad bearings the screech was so loud.
But when we got back into the woods there were thousands of them hanging from the trees coming out of their shells.
I thought it was really cool to see. They don't bite, they don't destroy anything, they just hatch, mate and then die leaving a meal for some other critter.
Read the 1st sentence in Diet:
http://en.wikipedia.org/wiki/Cicada#Diet
That sucking mouthpart can open an animals’ skin and leave a welt or sore, and it often gets infected. It’s not common to every Cicada, but some do. I’ve seen it firsthand, on a variety of hairless livestock, as a Farmer for 22yrs. Otherwise, they’re harmless, but I don’t like them. Shoveling the dead into compost piles is lousy work. At 600-1, we’ll need front-end loaders this cycle.