The vast majority of all online poker sites are very reputable and you should not have any issues with them. There is just too much money involved, for them to

the players.
Having said that, there have been some issues at very small sites with slow payments, etc.
If you stick with the
Poker Sites for US Players we recommend or the
Top 10 Poker Sites (which include non-USA rooms) I seriously doubt you will ever have an issue.
Jason