The H&R spring are good. But you will need to replace your shocks too - otherwise your OEM shocks will wearout in no time. Also the H&R springs make the car sit higher in the rear than the front. Raked. Alot of people go with the Eibach springs if you want a lowered look/car.
Or... get the BMW Performance Suspension kit from BMW (either Dan@United or Ticher BMW). The PS kit comes with springs(but only lowers the car 10mm!), tuned shocks and a front sway bar and all the misc hardware. $1,200 I think.