I had luck once, from $5 I got for free I made around $300! That was couple years ago, some new casino offered 5 dollars for trying their site! I was really lucky, I got some bonus very fast, after that another one. I played slots, changing them, and I was lucky to hit several more bonuses with good payouts, after some time of playing I had more than $400. There was a roll over if you want to withdraw, I played and lost some and in the end
I had $300 in my pocket!
Some casinos choose to give you some bonus with roll over, if you are lucky to wager that amount you can withdraw them, which is more fair in my opinion. Second way is that you can play with amount they gave you, but you can't withdraw more than $5-$10 even if you win more, that's stupid!