I had one for my '04. Started with STG1 and it really smoothed the powerband out, improved MPG, and made a slight increase in HP (BUTT DYNO, never on a true one so I can't verify the increase). Went to STG2 with a DP, 3rd cat eliminator, and an axleback. WOW.
I know they have really improved them since the model I had, so I highly recommend it...I probably end up getting one for this car too.
Check out Cobb's forums for a detailed description of the different capabilities and many firsthand experiences.