The first few seasons of this show were as funny as they were different from other TV shows. A group of ultra-nerds whose humor often involved actual scientific jokes made for wicked comedy for people who like cerebral humor. But the show had enough good acting and physical humor (think of Sheldon's expressions and voice) that just about anyone could find it funny, even if they missed the "inside" science jokes. Well, that could only last for so long as the show both started using up that kind of material as well as starting appealing to a broader audience. So in the fifth season, we see a move towards more traditional humor, while still keeping some of the great nerdness of the show.
The ultra-nerds are now mostly just super-nerds, as they are "normalizing" with girlfriends, more normal problems, and in essence, turning this show more into Friends than it was at the beginning. Don't get me wrong, the fifth season is still very much a creative and hilarious show that is full of science and nerd jokes that are sure to make you laugh out loud. Only the show is no longer purely about how awkward and unusual its characters are. Leonard is learning about women, Howard is mellowing out his creepy/bizarre "romantic" ideas, and Raj is increasingly able to speak in front of women (often thanks to a convenient drink in his hand). Sheldon is still mostly Sheldon, although his new girlfriend takes some of the edge off him. I quite like the addition of Bernadette who fits well with the existing cast. Amy isn't quite as good of a fit, largely because I think Sheldon was at least as funny, if not funnier, when he was single. Penny is still an excellent counterbalance for the group, although she now is starting to get, or at least know, some of the science jokes. Her relationship with Leonard is made more interesting by last season's addition of Priya, Raj's sister and sometimes love interest of Leonard. This makes Leonard a little less desperate than in the past.
So all in all, this is a very funny season to watch. Maybe not as funny and fresh as the original few seasons, but it's still my favorite current sitcom (yes, I hope it keeps going strong in season 6). If you are new to the series, I'd highly recommend you start with either season one or two, so you can get a sense of how the characters have evolved over the series. If you're a fan of the show, then this is a strong addition to any collection. I'd like to give it 4.5 stars, but seeing as how I think it's not quite as funny as the first seasons, I decided to give it four stars. It's still great comedy, just different from the original seasons that launched it. TBBT season 5 might no longer be a pure nerd-fest, but that might be a good thing insofar as broadening the appeal of this show, and it's still whole lot funnier than most of the other shows out there. An easy recommendation.