There would be several benefits of a scientific breakthrough that allowed people to live radically longer lives. For example, amazing scientists that have done a lot for the human race could live much longer and continue their research to further benefit mankind. Also, people could enjoy the company of their friends and family for a longer period of time.
However, personally, I don't think the benefits outweigh the potential negative consequences. There are many problems that would sprout from humans having a longer life-spans. For one, the economy of the US is already suffering, and it would only get worse if everyone lived for an extended period of time since the men and women that can actually work would have to take care of the elderly that would no longer be able to work. Also, the human population is nearing a very dangerous point. Eventually, the planet will no longer be able to support the entire human race. If people live for a much longer period of time, then that day will come considerably sooner, giving the human race even less time to find a solution. While people being able to live longer sounds nice at first, I personally feel that the long term affects wouldn't be worth it.
If I could choose my lifespan, I'd like to live to the age of 90. I'd like to live to the age of 90 because by that point, I'd have had time to enjoy my retirement but would most likely start to become bored since I wouldn't be able to do much.