I thought it was one of the stupidest shows ever at the start but I kept watching it for fun and once you get into the mood, it's pretty funny. You always know what's coming, the jokes are stupid, but you laugh anyway cause you know exactly what you're gonna get. A lot like Two and a Half Men like that, except take out the sex jokes and insert "nerd jokes" which aren't really "nerd jokes" at all but lowest common denominator nonsense.
It has fallen off SERIOUSLY in the last two seasons, once the nerds randomly got game and even Sheldon got a girlfriend (what the fukk?), and any type of emotion got involved. The show's premise ran out after three seasons, and since then it's been a lot of relationship nonsense.
The first 4 seasons are good, but last season (season 5) the show fell off. Now I'm thinking about dropping the show from my rotation & thinking about getting into another CBS multi-cam sitcom I've avoided for years, How I Met Your Mother.
As far as HIMYM, good times, VERY good times for about 6-7 seasons. They really did go as far as they could with the premise, then they started getting "relationshippy" (the characters started getting involved with each other) and the storylines just keep going in circles now, but I still keep watching.
Both shows are now half of what they were but I've watched em for so long, why stop now. It's the curse of the American television business, if these were Brit shows they'd have dropped 3 seasons of absolute fire and be seen as a classic, but they gotta squeeze every last drop out of these shows, and then squeeze some more...
Big Bang Theory = stupid comedy if you have nothing better to watch
How I Met Your Mother = smart comedy with some nice moments thrown in (think of a lightweight Scrubs)