I think it's perfectly rational to play the lottery, as long as you are completely aware of the odds and you are happy with the value of what you are buying. Same as with anything else you buy.

I buy a megabucks ticket about once every year. Sometimes twice. I buy it for a little bit of fantasy. It's totally worth the dollar. In reality I'd hate to win a big lottery because the evidence is pretty overwhelming that it would ruin my life (not the financial aspect, I could handle that bit, it's the social consequences that would make me wish I had never won).