I've gotten really lucky a couple times (triple 7's, and the like), but needless to say, I only play the slots if I have absolutley no money to buy coins, and there is a prize that I really want (usually a TM). I just wait until I have access to the Elite 4 to really worry about buying the expensive Pokemon like Porygon, since trying to get the amount of coins needed just from playing the slots is nearly impossible. Buying coins is much better suited for that.