I've constantly had thoughts like this. But instead of getting a power, it was just everyone being granted a wish, and if your wish would make it impossible for someone else to get theirs (for example, if you wish for everyone to die), then you get transported to an alternate reality where it comes true. My own wish was just to go back to my childhood years with my current skills, wisdom, and personality, so I could try to live a happier youth.
Back on topic, I'd probably want the ability to see into the future, becoming aware of all possible outcomes depending on how I act. So if I had to make an important decision, I could use my power to see the outcomes of each choice. I'd start off being able to see maybe a couple seconds in (which would still have some use), and as I use it more, that time slowly grows to minutes, hours, days, and maybe even years.
If I have to pay a price, I guess I could just benefit far less from my foresight when I use it for large gains, such as winning the lottery (so if winning would get me a million bucks, then using my power to win would just net me a hundred or two).
By the way, you sure this doesn't belong on /ot/? Not trying to sound rude or anything, I just get the feeling it's better off there...