If you're referring to PPPoker, I don't consider it completely reliable, since you rely on third parties to collect your prize. However, there are big clubs that work in this sector, and if they offer you the opportunity to earn a certain amount of money, it's because they are willing to pay you, and you won't have any problems. On the other hand, Party Poker is super reliable as long as it's not restricted in your country (don't worry, if that were the case, they wouldn't allow you to play, as is the case in Argentina).