I binged this show on Netflix and found it quite addicting, even when it became increasingly farcical in the later seasons. While the characters and story lines could be silly at points I have to say the acting was better than on most teen shows. Leighton Meester, Ed Westwick, Taylor Momsen and Kelly Rutherford were particularly strong. I loved the fashion, the NY setting, and the music. I adored the copious film references as well. While the non-stop bed hopping and relationship merry go round got annoying, one of the main couples has the best chemistry I've seen on any television show in years. These two are reminiscent of Rhett Butler and Scarlett O'Hara, a touch like Bonnie & Clyde and very Beatrice & Benedick. This one relationship really carries the show emotionally and makes it worth watching until the end to see how it is resolved.
There are a large number of negative reviews on IMDb so I also feel compelled to offer some warnings: It's not a realistic TV show. It's hyper-realistic. A fantastical soap opera if you will. If you're the type of person that gets annoyed over characters traveling from Brooklyn to the UES in 15 minutes don't watch. You'll go mad.
It's not a show with warm fuzzy ethical teen show protagonists like Sandy or Seth Cohen. I was a big fan of The OC. This show has some similarities but is much darker and much more satirical. The characters on Gossip Girl that you expect to be the moral center (the "poor outsiders") end up being quite devious while the clearly wicked rich bad characters end up being the most emotionally compelling. Every lead character is gray. If you need a show with clearly delineated heroes and villains don't watch. You'll hate everyone by the end.
It's not a show with a moral code. It neither reflects traditional conservative values nor progressive liberal values. If you need a show that promotes "good values" of either kind, I'd stay away.
It deviates wildly from the books. If that matters to you, stay away.
But if you want a glossy glitzy glamorous guilty pleasure filled with beautiful people like Chace Crawford and Blake Lively, tons of drama, fashion, and some over the top but emotionally compelling characters and relationships? Give it a shot.