A truly representative poll will show that, of the people that have played, slightly more than 50% have lost and slightly less then 50% have won.
I don't think so. Most people who have played have played more than once. The longer you play, the greater the chance that you're down not up.
A month or so ago I looked at the stats for Just-Dice.com. There were about twice as many losers as winners, and the house edge there is about half that of SatoshiDice at just 1% compared to SD's 1.9%, so I imagine the ratio of winners to losers is even lower at SD.