I imagine a lot of slots sites are "legit," but that doesn't make them a good deal.  That is, they may simply have bad pay out odds, even though they use a fair random number generator, and do pay out when you win.

I've investigated Party Bingo, and found that their games are "fair."  They do pay out when you win.  The winning numbers are generated randomly.  But they don't publicize the amount of money that the house takes out.  It turns out to around 30%, which is horrible.  Also, their progressive jackpots, though they appear "big," are actually small considering the odds against hitting (often in the range of 10-20 Million to One, per Bingo Card, or even worse!)

I think you'll find slot sites are often similar.  Yeah, you'll get paid when you win, but you'll be bucking some really rough odds.  Investigate whether the site you're looking at:

A) Publishes their odds

and

B) Is certified by a gaming council or comission, such as the IGC Interactive Gaming Council, or Kahnawake Gaming Commission.