What does this formula say?

It tells us which part of the sum of money the player should allocate for further games to maximalize the profit and at the same time minimalize the costs of the game.

Kelly`s criterion may not be enough for great losers. What if they have series of bad luck?

Kelly`s criterion ensures a safe betting strategy. The extent of the bet depends on the amount of money that we have. If our pockets are empty, the bets are low as well. When we win some money, we bet a bit higher. We will never go bankrupt because we do not bet everything but only a part of our capital, leaving something for further.

Nontheless, I would like to stress that the formula makes sense only when we have a little predominance in the game, more than its assumptions offer. The theory of probability is assumed to be unprofitable for the player at the very start.

Unfortunately, casinos do check and control whether the guests use any helping device. It would have to be "smuggled" into the game room and it is not that easy.

So, if we play fair, we have to rely on good luck.

Generally yes. There is no way we can help ourselves using purely mathematical rules if the game has-according to mathematicians- negative expected value of the winnings. That is the way the games are designed because gambling organizers want to make money not lose it.

You can buy those "wonderful systems" which offer the players the possibility of winning for certain.

Those methods are usually based on stake progression. We have to choose a number or a pair of numbers and keep betting that number until it wins. Obviously, we want to make a bigger profit, more than we put into the game. We have to bet more money next time. When we eventually come across a piece of good fortune, The winnings are far bigger than loses in the previous games.

Thinking logically this is a compact system- if we choose a number and keep betting it, it will eventually win. The trouble is whether we have enough money to bet more and more. The costs of the game grow higher and after a few loses in a row the amount of lost money can be absolutely huge.