I put in $15 dollars once and turned it into $100. I won most of that in about a four hour strech. After that session, I found myself loosing more than winning so I cashed out. I'm not exceptionally skilled at poker, but consider myself pretty well informed on poker strategy. Play money sessions are good practice, but in no way demonstrate how a real money game will play out. I've found the most realistic play money games are tournements because at least the players stand to lose their seat at the virtual table.
The average skill level of the online player has been increasing in Texas Hold'em due to its recently popularity. That said, there's still a sucker born every minute. Your best bet is to play only at full tables (10 people) and only play exceptional hole cards. It's really slow to play this way, but you're sure to turn a profit if you only play when you're sure you'll win.
According to others I know that play online more extensively, Omaha Hi/Lo is the new game of choice for winning the big bucks. Lots of people play Omaha without understanding how the strategy differs from Hold'em.