Absolutely love the show and have watched it since it started. My wife started watching it with me, and thought it was the worst comedy she had ever seen, but still watched it with me, now she can't stop! So it's good enough to win people over. (She has been buying me the box-sets as gifts now and then which is even better)
I love the fact that the science (even if I can't follow it) is apparently real (The whiteboards seen around the apartment always contain science in-jokes amongst the equations - not that I get them), the geek jokes, the brilliance of the characters - where else would you find a show where the main traditional geek character (Leonard), normally the comic relief is the normal character.
Pure genius, and one that is rapidly becoming recognised as such, thankfully, two major acting awards for Jim Parsons (Sheldon) The Golden Globe this year and the Emmy last year, and the show has just been picked up for another 3 seasons.
My favourite episode too many to choose from!