I dunno, it's been running for long enough that it has to be appealing to some audience. Personally, I rather enjoy it. I couldn't tell you why, it's just one of those things. I have friends that love it, and friends that hate it. The biggest problem I have with it (and it occurs in every show Lorre's ever had a hand in) is that, for the first couple of seasons, it looks like there's going to be some serious character progression, and then it just halts and starts making the same joke ad nauseum. In this case, as has been pointed out, it's "look how weird nerd culture can be!"
Here's the thing: when it opens, Leonard, Sheldon, Howard and Raj are all extremely socially awkward. Kind of a stereotypical trait for your nerds to have, but I'm cool with that. Penny is introduced, and we see Leonard attempt to try and fit in, necessitating that he shed some of his awkwardness for her. As the season progresses (up until their first date in the finale), he is trying "normalize" himself, and she is slowly coming around to the idea that nerd culture is a bit more diverse and engaging than she initially gave it credit for (see the episode where she immensely enjoys herself playing Halo 3).
And then... that just stops. Leonard has come out of his shell a little bit, but the show becomes fixed around the relationship between he and Penny, where he is still the stereotypical nerd, and she is the girl who doesn't understand nerd culture past a certain point, and that's where all the humour comes from. There's a similar occurrence with Howard and Bernadette, though to a lesser extent because she herself is a bit nerdy (really, the only thing she needs to become accustomed to is how terrible a human being Howard is. Seriously.), and then again with Amy and Sheldon (when she has to deal with the fact that he don't like touching). That one bugged me quite a bit, because Amy is supposed to be the female equivalent of Sheldon, but then she starts hanging around Penny and she becomes more of this awkward in-between thing, which sort of defeated the purpose of introducing her in the first place, or so I thought.
But at the heart of it all, every single character who makes an appearance on that show, save for Penny and those connected to her (her father, her friends, her non-Leonard boyfriends) are nerds, and Penny has to deal with that. You know how every good TV show needs conflict? That's the conflict here: Penny is non-nerd, must learn to contend with nerd things. That's what it started as, and it never progressed beyond that.
So, yeah: the show initially threatened character progression, and maybe a fresh outlook on its original idea, but scraps all that in favour of a "nerds are weird, look at Penny try to deal with it" joke that runs on and on through every season. Again, I like it, but I'm not particularly difficult to please when it comes to this sort of thing. But as far as the construct of the show goes, it's sloppy, one-dimensional and tired. I'm sure a lot of "nerds" don't like it because it more or less belittles their culture, and anyone who's really into TV shows wouldn't like it because it's not particularly well-constructed.
WOOOO! Essays!