Over the last 10 years, I've been using Kool-Stop Salmon brake pads. They worked so well that I don't mention them, not even in the substantial information packets I provide to fellow cyclists on grand tours.
Well, during the recent Tour of Hokkaido, I once again forgot to mention them, and during one rainy descent, I commented on how smooth and easy Japanese descents was. No one else agreed with me, to my surprise! Well, it turned out that I was the only one running Kool-Stop Salmon pads. Everyone else was running the OEM pads (not surprising --- it takes a long time to wear out brake pads in California), and those were apparently not stopping the bikes, thereby giving my friends a scary experience.
Come to think about it, during the 2007 Tour of the Alps, and 2008 Tour Across France, the only time I was beating my companions down the hill was when it was raining. Again, I was the only one running Kool-Stop pads. Even if you don't ride in the rain, there's still a good reason to replace them --- here's a comparison between the Shimano OEM pads versus the Kool-stops. Yep, those OEM pads tear up your rims, so your wheels will last longer with the Kool-stops than with the OEM pads.
Given that they're only $6.95 on Amazon.com with free shipping on purchases over $25, they're a cheap and easy upgrade to your brakes, and pay for themselves on the first rainy descent.