What is the paper made of?
Like most higher end photographic "papers", the true difference isn't in the paper itself at all. It's in the coating on top of the paper base that actually absorbs the ink from the printing process (inkjet paper) or in the photosensitive layers on top of the paper (lab photo paper).
In the case of what is often referred to as "Metallic Paper", "Semi-metallic Paper", "Metallic Satin Paper", etc. the coating includes a mylar layer between the layer that contains the photosensitive emulsion and the paper substrate for lab photo paper. For inkjet photo papers the mylar layer is just below the top layer that absorbs the ink from the printer.
Please note that "metallic" papers are different from true "metal" prints. In the later case the photosensitive emulsion layers or dye receiving layers are bonded on top of actual metal sheets, usually made of aluminum.
What type of photographs is it good for?
Metallic paper is good for very high contrast images. This can range from high contrast Black & White prints to prints of High Dynamic Range images, either color or B&W. Here are a few examples of images that I've been very happy with when printed on "metallic" papers. More than any other type of paper I've found, when displayed properly lit the "semi-metallic" papers look like there is light illuminating the image from behind, just as you see when viewing it on your computer monitor.
Traditional prints of the same images look dull in comparison. Although this is a simulation, here is about what that last image looked like printed on traditional gloss photo paper. (This simulation still looks better than the print I tried to match.)
Display conditions also affect whether the use of a "metallic" paper is a good choice. The metallic print needs to be displayed under good, diffuse lighting. If displayed in a darker setting with narrow direct lighting, such as a single overhead accent light, it doesn't allow enough light from different directions to get to the mylar layer under the color and reflect back through the emulsion or dyes. Although they can look very good under glass, "metallic" prints also tend to look even better with no glass in front of them.
Conversely, "metallic" paper is not usually a good choice for photos where accurate skin tones are a priority and should generally be avoided for most types of portraiture. An exception might be an environmental portrait where depicting the high contrast or metallic looking surrounding is a priority over natural looking skin. Perhaps a welder who is covered in protective gear anyway
in an environment with lots of shiny metal and sparks flying would benefit from a "metallic" paper treatment. The portraits of the bride & groom with their families and wedding parties would not. (And yes, I have seen such portraits printed on "metallic" paper by others and it wasn't pretty.)
How is the quality of the paper?
Like any product, it depends on who makes it and the attention paid to quality control during its manufacture. The "metallic" papers from recognized leading photographic paper companies, such as Fuji (Pearl) or Kodak (Endura), can be quite good and the prints made using them can be stunning when the right type of image is used and the prints are properly displayed.
How does it compare to the other types of paper used for photography?
Where does it sit on the scale of paper types (high gloss, gloss, semi gloss, lustre, matte)?
"Metallic" type printing papers are high contrast and very high gloss.
How well does it work in the high-end printers (ex. Canon Pro-10)?
Like any mid-grade inkjet photo printer such as the Canon Pro-10, it all depends on matching the correct paper, ink, and printing profiles. When done properly the prints can be very good.
I personally prefer "metallic" prints lab printed on photosensitive paper to "metallic" prints printed on inkjet papers. But then I also prefer more normal types of lab prints on top grade photosensitive paper to more normal types of prints on top grade inkjet papers.
