Having colleagues and friends in the UK, I can agree with Simon when he says that its an insulting gesture in the UK. I also agree about its associations with WWII.
However, in the US, the "V sign" is simply a peace sign. Many people associate it with the hippie movement in the 1960s in the US. Back then, it was a symbol for anti-war (peace). It also has some associations with US hip-hop culture.
Today in the US, it still means "peace," although most people aren't thinking about wars when they use it. You'll mainly see someone use the gesture in selfies or group photos and its still seen in hip-hop culture. Having studied in Taiwan, I can say that the sign is used a lot more frequently in Taiwan than in America (not sure how that compares to China). In selfies/group photos in America, I would say most people use it lightheartedly--you'd probably see someone using it in a "funny photo" where people are making silly faces, have their tongues out, and other funny things. If I were to take a photo with a friend, I wouldn't use the peace sign unless we were taking a funny photo, and even then, I wouldn't use it all the time.
All this to say that there is a major difference in how this sign is used today in the UK and US, even though we are both English-speaking countries.