I began watching this sitcom after its run had concluded, and I consider myself lucky to have experienced all 12 seasons sequentially. I don’t share the opinion expressed by another reviewer who suggested the series declined in quality over time; instead, it strikes me as consistently excellent, much like other shows produced by Chuck Lorre, which seem to function as a comedic counterpart to Aaron Sorkin’s dramatic works. I did encounter some minor drawbacks while watching. Sheldon's personality and actions occasionally became tiresome, but this was offset by the gradual and deliberate way his character developed. Likewise, I observed believable development in the attitudes and behaviors of all the characters involved. The writing generally maintained a high standard, even during episodes that felt less successful— those where the storylines simply didn’t resonate with me. It's remarkable to me how the show managed to avoid the pitfalls that affect many long-running programs, like what happened with Happy Days when they altered the Fonz’s character into something resembling a superhero who could accomplish virtually anything. It’s amusing, actually, because I know someone who couldn’t bring themselves to watch it because of the laugh track, so I was surprised to discover that there wasn't one—the show was filmed in front of a live audience. It’s hard to believe!

Source on TMDB