It is. They all are, of course. But I read a mathematical analysis recently where the author said that once the Powerball hits 450 million (I think that was the number), the math says it's worth it.
The odds are 1 in 292 million. Since tickets are $2, you would have to spend $584 million to get all the numbers and guarantee a winning ticket.
But, you have to remember that you don't get all the jackpot... you get the annuity amount. Then there are taxes. And, there's always the chance you have to share the jackpot with others who got lucky.