Anyone capable of reliably counting cards (normaly 4 decks) and is not illegal, though obviously gaming establishments only like practices which favour the house. Hence the stories of card counters being thrown out and often given a clump.

But card counting only gives the player better odds and help predict the likelyhood of being dealt strong cards, and so bet accordingly.

If you can resist the greed 'Demon' then you can get away with it, like i'm certain 1000's do regularly.