Johnny Galecki (David from "Roseanne") is good, but Jim Parsons, the taller, even geekier one, is just hysterical.
The plot of "beautiful dumb blonde lives across the hall and geek guys try to impress her" is not very original, but if you just ignore Penny, the two leads and their two goofy friends are great. It's a little bit like if "Harold and Kumar" had a TV series, only Kumar, at least, was waaaay cooler than these two.
Some of the noteworthy quotes on IMDB will give you a sense of the show's flavor.
And if you love "Roseanne," pick up the first season of "Big Bang" on DVD. You'll see not only Sara Gilbert (Darlene), but Laurie Metcalfe (Jackie) make guest appearances.