If you were wondering what to expect from the Street Fighter IV Femme Fatale Costume Pack, here's a video to give you an exact idea. The pack runs $4 or 320 Microsoft Points.
The pack includes costumes for Cammy, Chun-Li, C. Viper, Rose, and Sakura.
I might pick up the pack that includes costumes for the characters that I play. It's kind of cool to look different from every other player. Also, last night I fought a Ken player that didn't Shoryuken once... it didn't make any sense.