Pros: Meringue powder protects against salmonella. Has a long shelf life.
Cons: not as tasty as fresh egg whites for Royal Icing
Meringue powder is not just dehydrated egg whites. It is a mixture of pasteurized and dehydrated egg whites, sugar, salt, vanillin and gum. This mixture comes in a very fine powder form. When beaten with water and powdered sugar, it creates an icing the same consistency as if made with fresh egg whites. This is the kind of icing you will find in cookbooks listed as Royal icing. It has a wonderful white color. It dries hard with a matte finish. It can also be easily colored. This is the stuff the professionals use for frosting cookies and cakes. They also use it for piping and for making flowers, borders and lettering.
Traditional Royal Icing is made of sugar, lemon juice and raw eggs. Due to concerns about salmonella in using raw egg products, many bakers are now opting for meringue powder. I personally find that icing made with meringue powder isn't quite as tasty as the stuff made with raw eggs. To counterbalance the deficiency, I'll add a little extra extract (vanilla, lemon or almond) when making up a batch of icing.
It is essential to keep your icing covered as much as possible when working with it. It dries out very quickly. I will usually push the plastic wrap right down onto the surface or drop a round waxed paper on top. Otherwise, a crust can form which is not tasty. Another way to keep a crust from forming is to add a couple of drops of glycerin to the icing. Glycerin is available from cake decorating supply stores and party stores. Cookbooks will give you more advice on making royal icing and getting it the proper consistency.
My recipe for Royal Icing with Meringue Powder is to beat the powdered sugar (about 4 cups, 440 grams, about a pound) and meringue powder (3 T, 30 grams) until mixed. Then I add 1/2-3/4 cup warm water and 1/2 tsp of vanilla, almond or lemon extract. Next I beat on medium to high speed until it gets very glossy and stiff peaks form. That will take about 5 to 7 minutes depending on the power of your mixer and the weather. If necessary to get the right consistency, I'll add a bit more powdered sugar or water. I know I have the right consistency when I lift the beater and the ribbon that falls into the bowl remains on the surface of the icing for several seconds. The icing needs to be used immediately, or moved into an airtight container. You should get about 3 cups of icing out of that recipe.
Kept in a cool, dry place meringue powder has a very long shelf life. That said, unless you are constantly baking and decorating cakes, 10 pounds of meringue powder seems, to me, extreme. I will decorate a few cakes a year, and a package that large would last me more than a decade. Meringue powder is a terrific product for its purpose, but isn't good for anything else. If you are a prolific decorator of cakes or cookies, you might go through this faster. If you do that much decorating, I suspect you already know about this stuff.
So high recommendations on the product itself, but I would seriously be on the look out for smaller containers.