Really depends on which game you are playing, over time the house edge usually tends to pull back any short term winnings. I think that if you play a PVP game such as poker and its variants then you have a fair chance of winning if you are playing carefully and with tactics. Definitely stay away from games based solely on luck as you will end up losing in the long term.
I once saw a document about casinos in Vegas and they were saying that the casinos aren't making the most money on high stake poker or blackjack but on slots. The less skill the game requires and the less rules it has the more people will play. And slots are made for people to lose. When they play they don't know the odds and are attracted by light and sound and the real money, not chips being thrown at them. Online dice work just like those machines.