All the cool stuff I'd like to do would probably change history to the extent that'd I'm never born and thus can't change the past with a time machine. But then history is back to normal and I'm born. Then I try to use the time machine to alter history, and once again I succeed, but as a result I'm never born. I then can't change history with the time machine. But I can. Because history is once again normal because I never changed it.

If the technology is even possible, I wonder if this is why we never see time travelers visiting us from the future.