I have had some success at the casinos here in Connecticut with a conservative variant of the martingale system, and would like to share my thoughts.

Many gambling experts will tell you it doesn't work, plain and simple. I'm not an authority on gambling and know little about mathematical statistics, but respectfully disagree based on my own experiences. You can use the martingale system and almost definitely earn a decent return with blackjack, but only if you stop very early on. I take a small amount of money  and bet 10%. If I lose, I bet 20%. If I lose again, then I bet 40%. If I lose again, I walk away. So really I only bet 70% of my original bankroll.

When I win three or four times-- just that small amount-- I also walk away. With a $200 ($140) bet, that's only a $60 winning... but still, a 30% return beats anything you can find anywhere else. Even taking the gambler's fallacy into account, the odds of losing three times in a row without winning at least once are small. Try blackjack solitatire and see what I mean. If you are taking a larger bet, say $500 (effectively $350), then winning once is $50-- not bad. The odds of losing three times in a row before winning at least once are small. You can go with a 5-10-20-40% bet as well, increasing your chances of winning once even more (you would have to lose four times in a row).

The fundamental problem with the martingale system is not the system itself, but that people get too greedy. You eventually WILL lose three times in a row, often more. It is hard to walk away when you lose, because you want to recoup your losses, but then of course you almost always end up losing more.

Play regular blackjack, which for non-card counters has better odds than Spanish 21. Don't double down or split hands except in "extreme" circumstances. If you play this very conservatively, at least by my experience, you stand a decent chance of winning a small amount of money, which is always better than losing.