I cannot believe that people are debating about the cost of warranty and going on about how much you can save by putting the money in a "emergency pot"...
Imagine your front calipers seized solid as the dust seals have perished and let water corrode the pistons. Nothing scary, that's what your kitty is for. Thus you promptly go to BMW, buy two rebuild kits for £16+VAT each and then rebuild them yourself. Hmm, something is missing, ah yeah, pistons... After a day trawling the internet, you find someone who sells separate pistons, but he is in the USA. You order the kit for £100 for both front pistons, includin shipping, duty and VAT. Now you have to wait for the muppets from the Post Office to deliver it and charge more for a "handling fee". Damn, it is late December and it is quite cold and wet outside. Never mind, the kit is here after two weeks of waiting, you will just spend the whole day outside, with the car on jack stands, wheels, discs and calipers off. Damn again, you need compressed air to blow those pesky old pistons out of the calipes. You ain't got a compressor. Improvise, McGiver... Somehow let's assume that you have managed to rebuild the calipers somehow. Now you have just realised that the discs and the pads have gone too because of the excessive heat due to sticking calipers. Damn, damn and damn again. Hello, is that ECP? Yes, we have them in stock, that would be £300 + VAT for 2 discs and front pads. Yes, when can you deliver? You say that next day is another £15 extra? Ah, I see. You have everything finally. Never mind, that you have wasted a whole day of your weekend. You start putting it back together and you realise you need to bleed the brakes. Back on the phone to ECP for ATE Super Blue, and then to a mate - after all someone has to pump that pedal. Maybe on Sunday night you are done, full of great satisfaction that you have beaten the system... Until next time, when something more expensive goes pop!