Ruy Lopez, Marshall attack, are you afraid of it?
You actually should be (frankly, I am afraid to play it for White). Two world champions played/play anti-Marshall lines to avoid it - Gary Kasparov, Kramnik, same with quite a few other GMs , why?
Kramnik vs. Leko, 2004, World Chess Championship. Leko as Black played Marshall attack 4 times, 3 times Kramnik avoided it, drew, 4th time accepted, played a novelty - Leko found a hole in it and won.
Recent game, Anand-Aronian, Morelia-Linares 2008, final blow, 30. ... Re2-e3! (31. fxe3 Qxf3):
GMs Adams and Nunn like to play it as Black, the result?
Adams - 29 games - 8 wins, 4 losses, 17 draws, Nunn - 27 games - 7 wins, 3 losses, 17 draws.
So, what is it, how it appeared?
Frank Marshall first introduced it again Capablanca in 1918.
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Be7 6.Re1 b5 7.Bb3 0-0 8.c3 d5 (The Marshall Attack)
9.exd5 Nxd5 10.Nxe5 Nxe5 11.Rxe5 c6 ( the best move, Marshall played 11. ... Nf6 )
followed by 12. ... Bd6 and then often 13. ... Qh4. White's king side has no defenders and as Marshall himself said, "the attack gives Black many winning chances and should be good for at least a draw."
Here is the my best game so far using Marshall attack, played recently on FICS:
( 12. d3 Bd6 13. Re1 Qh4 14. g3 Qh3 15. Nd2 Bg4 ):
After 15. ...Bg4 white plays 16. Qc2, this allows combination ( now I understand why white in many variations exchange bishop on b3 for night on d5) 16. ... Nf4!
White resigned ( 17. exf4 - forced ... Bxf4 18. Nf3 Bxf3 with mate on g2).
Playing it online, I have so far 22 wins, 7 losses, 4 draws, 73% score. Because a few of the opponents had lower rating than me, the more exact characteristic will be my performance rating. It is 120 points higher than my average rating ( meaning I play like I have rating higher than mine by 120 ). For comparison my performance rating in Benko gambit is +25.
Of course, Marshall attack is not a recipe for guaranteed win, there are many good defenses for White ( by the way Anand has good results with White, but your opponent is not him, right?), so you take risks, but if you want to get an attack with Black pieces just after first 16-18 moves, where for your opponent every move is like crossing the minefield ( step aside of the safe path and you are blown up - see above ) you would definitely try it.