Lisa, I've used Color Mechanic, and while it works, I think one can get the same results using either Hue/Saturation in the manner Jonathan described, or the Selective Color adjustment in PSCS2. I didn't find Color Mechanic all that it's cracked-up to be in terms of really isolating the color you want to adjust. But one other observation on this matter in general - I have often found that if there is a problem with one colour, it often means that the overall colour balance is somehow off, but is more noticeable with one colour than any other. Hence when I spot this issue in some of my images, the first thing I do is verify the overall colouor balance before resorting to selective colour adjustments. Of course, this doesn't apply if the objective is to intentionally change a correct colour in a balanced image to something else for whatever reason.
Mark: It sounds like you're expecting something different from Color Mechanic than I (and the original poster) are. I'm not using it to correct for a color cast; if that's what you're using it for (and that's what it sounds like from your post), then you're right, it doesn't work particularly well, but that's not what it's for. What I'm using it for is to modify certain specific colors (without changing other colors) where "reality" isn't exactly what you want, and that it does in a very convenient, quick and easy-to-control fashion. For example, using it to make a bland pale blue sky a more saturated blue, or to make brownish-looking vegetation look a healthier green. In my experience, it's very good at isolating a particular color. How well it works must depend on what sort of images one is using it on.