I'm with BMWROCKS. I have the sport package and my rears gave out at about 20K. The fronts had an uneven wear pattern, so I went ahead and had all four tires replaced at the same time ($1200). What's strange is that I'm at 32K now and the tires seem to be holding up better this time around. I'm hoping to make it to 50K before replacing them.
JonnieTran, you must be doing some SERIOUS pedal mashing to get your rears to wear out at 13K miles. Regardless of what's going on, I hope you're having fun. This car remains the best I've ever owned. I'm just over a year into ownership and am still blown away by the sense of confidence I have behind the wheel. Power, handling, yadda yadda....