IIRC my pair is from spring 2010. Still going strong. I should probably add a picture of what my brakes have to endure. The rear has been completely covered in mud and sand for a couple of months now.
I can't say that I've noticed any degradation over time.
I haven't performed any scientific tests and I'm probably not the world's best mechanic but I think that for me Shimano brakes perform slightly better. But apart from when using my Edge rims in the wet on gnarly roads it's not a problem. Right now I'm using alu rims and as long as I adjust for pad wear they work well even in the wet.