Surprisingly, I was late to the The Big Bang Theory party. But now I'm here and so happy that it's just started syndication so I can start the series from the very beginning (of course I'm still keeping up with the new episodes!).
I'm here to tell you those people really exist. I know because I've met them. In my own family. Seriously: I definitely have a Leonard, and if you took three others, took them to CERN and smashed them together in the Large Hadron Collider you'd get Sheldon (and yes, I know that the LHC wasn't created for that purpose - call it comic hyperbole).
They walk among us... but they're pretty harmless.