Crikey! Okay, so first off, until the light that comes on in the gauge cluster, which indicates the brake pads are at minimum thickness, your car does not need new brakes. Second, the CBS does a pretty good job at estimating how much pad life (in miles) is left until the brakes will need replacement. This all is assuming that you have not done anything to the brakes up to this point, like changed out to a new set of pads mid-cycle, or short-wired the pad-wear sensors.
I've never understood why dealers will try and pull off an early brake job when, if the car owner is a half bit educated (i.e. has
), would know that the car will indicate when the pads are due for replacement.
Third, it has been my experience with BMWs over the last 25 years or so, and doing all my brake jobs DIY, that it is best to replace brake rotors when the pads need replacing. BMW rotors rarely make it through a second set of pads before they get to, or below minimum thickness. So when you do need a brake job, it's a good idea IMO to replace the rotors. It's really only extra cost ($150 for OEM rotors at retail price) for the parts since a proper brake job includes removal and reconditioning of the brake rotor surface when a new set of pads are installed.
My recommendation is wait until the car tells you the pads need replacement and get the brakes relined at that time. Once the indicator lights, you have plenty of time to get the work done because the pads have several thousands of miles left until the brake would become unsafe. And, yes brake replacements are not rocket science and are easy to do DIY, if you have the proper tools and experience. But if you have no wrench turning experience and have no desire to work on your own car, then leave a brake replacement job to a professional mechanic. Any BMW independent repair shop can give you a reasonable price that is much lower than the dealer (in most cases).
And as pointed out, brakes will generally not "feel" any different up until the time the pads need replacement unless a rotor has become too thin and possibly warp, or the pads have disintegrated to the point where there is no friction material left on the pad's backing plate and the brake is known as being in a metal-on-metal condition, which in the case of an E9X would be months after the CBS has notified the owner that pads are due and the CBS annoyingly has pinged the notification sound at every startup and shutdown of the engine. However I have noticed with my '06 325i, that when the brakes are near needing refurbishment, heavy braking activity can lead to a bit of brake fade because the pads and rotors are a getting thin and have a difficult time dealing with heat bleed-off, which causes the brake fluid (that also may be near it's 2-year replacement cycle) to boil and reduce braking effectiveness, but this is only after really aggressive street driving.