I primarily use raw egg whites, which I feel have a superior taste to any meringue powders out there (or at least the 3 or 4 varieties I've tried). However there are food safety issues (potentially) associated with using them. (NOTE: I do not serve the public.)
I disagree with MEUG wrt the "inferior quality" of royal icing made with raw whites. I'm not sure what she means by that exactly, but I've done side-by-side comparisons of my recipe made with regular whites, pasteurized whites, and various brands of meringue powder and have found barely perceptible handling and drying differences. The primary differences are that those with meringue powder are usually sweeter (as meringue powder often contains additional sugar) and gummier/spongier (due to gums, starches, and other de-clumping agents that are added to them). And those with meringue powder are less prone to separating due to the additives, but this is a non-issue for me because I typically use my icing the day I make it.
But at the end of the day, I find the variations pretty darn interchangeable. What seems to affect the quality of my icing more than anything is whether I quick-dry it (i.e., with a dehydrator).
Also, without knowing the composition of other ingredients in any recipe (in this case, water and sugar in addition to the albumen in the egg whites), it's hard to make side-by-side recipe comparisons. For my tests, I try to control them as much as possible by varying only one ingredient at a time, and using exactly the same quantity (by weight) of any substituted ingredients. But try as I may, even these tests are not perfect, primarily because meringue powder has so many additives and the exact quantities of those additives are not given on product packaging.