A couple of years ago, we went away for the summer, and left our 2 daily drivers parked side by side outside. When we came back, on one of them, all of the brakes were frozen with rust, and not salvageable. Don't worry, it wasn't a Porsche. The other, my XC70, had only light surface rust on the rotors.
Instead of going to the dealer for new rotors and pads, I decided to go aftermarket. The parts looked pretty high quality, and all 4 wheels were under $200. A bargain! The fronts went in without problems, but with the backs I had some difficulty. I ended up having to hammer the pads in lightly for them to go into place. I didn't think anything of it, but a few times when coming off the highway, I noticed an overheating brake pad smell. I assumed it was the caliper pins, but disassembly showed no problems. The smell went away after some days.
Yesterday, after only 20k, the rears started grinding, so I went back to my aftermarket source for new pads and rotors. This time, putting in the pads required more than light hammering. That couldn't be right and then it struck me - the pads must be able to move for them to float correctly and release. Examination of the pads showed where they were tight, and I filed the backings so they could be slipped in with only finger pressure. A test drive showed that the brakes worked perfectly. I've always thought that on a modern car, brake work is a cinch. Changing pads on my 911SC can be accomplished literally in minutes. This experience was both satisfying and humbling.
The other question, though, is, why do the rotors on some cars rust, and others not. Old cars seemingly can be left outside forever, and show no major problems. Some modern cars rust very quickly. The rust seems to occur at the junction of the rotor and the pad. On cars susceptible to this problem, pushing in the pistons to leave a gap between rotor and pads helps a lot. Unfortunately, when you do this, initial application of the brakes has the pedal going to the floor. I leave a note on the seat that says "no brakes", without explanation. I've asked experts about this problem, but have never had a satisfactory explanation. If you have an opinion, why not leave a comment to that effect.