Martingale is definitely one of the most widely known and most successful roulette strategies (for some it is the best roulette strategy), and it is very simple to apply. It can be used only for outside bets, those that provide almost 50% chance of winning, and excluding the 0.

What Einstein actually meant was that there is no mathematical trick that can help you win at roulette. Each spin is an independent trial and, in the long run, the casino will win. This is different to a game such as Blackjack where the probabilities change as cards are dealt.