I've been a big fan of the Men In Black movies so I had high expectations for Men In Black 3, especially seeing it in 3D! Story wise, it was a great movie. However, from a pure fun aspect it was just ok for me. Will Smith and Tommy Lee Jones continue to be a great team but not as funny as in the previous movies. I found Men In Black 2 the funniest. The original will always be my favorite, though.
So, should you spend the money to go see it? If you're a fan of the series, then go see it. Just don't have high expectations and you'll enjoy it.