I never gave a lot of thought to racks, as long as they seemed well designed and didn't break. However, a couple of years ago I splurged and got a super-pricey Tubus rack for my commuting/light touring bike and I was amazed at the difference.

First, I bought the lightest possible rack I could get that was rated to carry the loads I expected to carry (it's a light-touring bike, not a super-heavy-touring bike). So the rack is light.

But the other thing is the rack is really *strong.* It doesn't flex under weight, and it even seems to stiffen-up the rear of the bike. I usually commute with one very heavily-loaded pannier (clothes, a laptop computer, usually some other stuff) and I have a hilly commute. I can stand on the pedals and crank up a hill and not feel any "swaying" of the load in back.

I never thought racks were that big a deal but my next rack will be a Tubus as well.