Color Range has two BIG problems... it creates a hard-edged selection, and usually things are only partially selected.
First, even with fuzziness and gaussian blurs applied, it's what I call a "hard-edged" selection.
Imagine a blue sky behind a dark mountain. If you select the sky with color range, you'll still have the problem of making sure ALL the sky was selected, and NONE of the mountains. This might work in a small print, or with a small change to the sky. But, in a big print (or with a big change), you'll see artifacts where your mask did not exactly fit. (And it rarely does.)
(If you want to change a blue sky, try a Selective Color adjustment instead, or a tonal selection! Select the sky AND some mountain, lock down the mountain, and have your way with the sky. Best to put the curve into luminosity mode to avoid weird color changes).
Second, color range usually always gives "partial" selection of areas...
Image a bunch of leaves against a darker area. You select the leaves with color range, using all the right techniques (using the "plus" dropper with multiple clicks, previewing with different selection previews, etc.) If you then examine the mask closely, you'll see areas of the leaves that are not fully selected (usually darker or lighter parts are not included in the mask). If you then use a curve to darken your selection, only the selected parts of the leaves will change. It could be you bring down the lighter parts, and they then match the unchanged darker parts, resulting in...tonal constipation! (The results that tlooknbill described).
Using "tonal selections", I (fortunately) rarely have to use color range. But when I do, I find it's helpful to run several filters on the resulting (usually crappy-looking) mask. With the mask selected, I use Filter:Other:Maximum at a radius of one. This helps even out the partially-selected areas. Then you need to smooth it a little with Filter:Blur:Gaussian blur, with a radius of 1-2.
But why go to all this trouble? If there's a tonal distinction between the areas, try my technique. Like I said, sometimes it doesn't work at all. But you can learn where it will be effective, and it's quick and is artifact-free. Life without hard-edges masks is so much more productive.
By the way, I do like the idea of using the blend if sliders to control the tonal selections, too.