At chess.com someone who uses the French opening asked if it would be worth it to learn the Sicilian Defense. This is my answer and I sneaked in a plug for lichess.org hoping the idiots at chess.com (who charge extra for everything lichess offers for free) don't censor it.

Sicilian Defence, Najdorf Variation. Modern Chess Openings calls it the "Cadillac" or "Rolls Royce" of chess openings. It's a lot to learn and also you would have to learn every variation White can use to avoid the Najdorf. Lots of work but it's worth it. The games are exciting. And if you somehow survive the Middle Game you usually have an advantage in the Endgame.

Best chess advice I ever got: Keep playing the Najdorf no matter how many times you lose with it. The advice paid off because now it's my favorite opening for Black. I occasionally get wiped off the board with it, but I still love it. To help learn from my mistakes I use computer analysis (free at lichess.org by the way).
http://www.chess.com/forum/view/chess-openings/should-i-start-playing-a-new-opening