In general I would beware and probably stay far away unless you know what you're doing!

I believe the first such of these sites was Swoopo that made its rounds in Germany and spread to other countries. Seems like some other sites realized they are printing $$, so they're copying their model (or maybe Swoopo is behind these too? Not sure).

Anyway, this is a game of nerve and sunk costs. Keep in mind that most people will spend money and get nothing. One person will get the payoff. There is some controversy whether or not this should be called "gambling" as you are spending money for a small chance to get a payoff. On a more theoretical level, you are taking on more risk for probably the same expected value of just buying the item, so in that sense it is gambling