I really enjoy this show. Only discovered it earlier this summer and we've just worked our way through the first 3 seasons. I think people laugh at Sheldon because of his social inepititude, I laugh at him because he says the things out loud that I'm saying silently in my head.
The only part of the show that irritates me is in the writing where, when the geeks are talking fannish stuff to each other, they have to reference it for the audience in ways that geeks just don't do when they're talking amongst themeselve, or the way that no one does when sharing common frames of reference. That aside, it is amusing.