Using "frak" as a mild curse word sounds really cool and sci-fi when you use it in your head, but not so much when you say it out loud.
In your head:
"This frakkin' traffic is about to drive me frakkin' crazy!! What the frak are these people doing??" (sounds hip and edgy)
"When the frak did gas go up another 10 cents???" (gets you strange looks from Weigel's store clerk)
I'll just keep that one to myself...