Well first of all, there are certain irregularities in the software of online poker. You will see MANY MORE FULL houses and 4 of a kinds in online poker. You need to ALWAYS be aware of that when you play and you suspect that you have the "nuts", but actually lose to 4 or a full house.
Another difference between live and online, is that you can pretty much bank on that flush hitting after seeing 2 hit the board at least 2 times more than in live play.
I have had good success playing online. But I have lost $300-600 in a night as well. My best win was $12,000 in a tournament at poker stars. The buy in was $200 for this event though, so dont think that the win came cheaply. Since then, I have made the final table in smaller events quite a bit, but nothing that time. Other first prize winning amounts have only been $1000 and $750. It has been fun, but except for a few big wins, it hasnt been real profitable. I am up a nice amount, but without the tournament play, I would not be up so much