Still trying not to reveal plot details before the jump: the concept is trite and cliché and while it hasn't been used in the 'MIB' franchise, it has been done everywhere else. There are a few television shows and movies based around the entire idea.
I mean, come on...time travel? The only saving grace is that Jermaine Clement is undoubtedly going to be awesome as the villain. He even looks like an alien. Josh Brolin does look a little like Tommy Lee Jones, at least enough to satisfy movie audiences. I'm sure it will be a nice summer popcorn flick, provided they don't screw it up.