If you're rain man and you can remember all the cards in a bunch of decks, you've earned the money as far as I'm concerned.
You don't remember the actual cards played -- you're just keeping a running relative total of high cards vs low cards that have come out. So, it's not really very hard, but it does require constant attention and takes the some of the fun out of playing. The power isn't in knowing how to play your hand -- strategy, i.e., whether to hit, stay, split, double down, etc, stays almost exactly the same regardless of the count. The power is that the players' odds increase when there are more high cards remaining to be played, i.e., when a disproportionately high percentage of low cards have already been played. The goal isn't to change how you play, it's to change how you bet. When the deck is "good", you bet 10 times as much as when it isn't, which is why it's so easy to spot a card counter. A six deck shoe is no harder to count into than a two deck shoe, but the information is less useful, because having 5 "too many" low cards is meaningful halfway through 2 decks, but much less so halfway through 6 decks. So, there's no need to "cheat" to count cards, you just have to have a high capacity for mindnumbing boredom. The teams that made a lot of money counting cards (and they were cheating because it's an individual game) have one person just counting and betting the table minimum, and if and when the deck gets good, he signals a new player to come to the table making big bets. That pattern was much harder for casinos to identify, and so it was successful for a long time until they caught on to where the losses were coming from. They almost never kick out an individual gambler even when it's obvious he thinks he's counting cards. Atlantic City is famous for kicking out, or otherwise hassling, players who are on a winning streak, just to break their "mojo". The science there seems dubious to me, as I think they'd prefer to let you feel hot and slowly lose back the money, which will happen of course. Vegas tends to let things just shake out, as long as they feel sure you aren't really cheating (e.g., through team play, using a device, etc.).