JackpotJoy are genuine, I used to be an afilliate for them.
You just have to remember, all games are against the computer, just like a fruit machine, they are not fixed, but they are set to only pay out as and when they say, for example nobody knows when the jackpots are going to drop, they are programmed to take a % then pay out.
So if someone has just won £1,000 on game and you join then chances are you will lose.
At least that his how I am lead to believe it all works.