I just got my aviation prints back from MPIX labs on Kodak Metallic Paper. John Rank and I worked on a project for National Association of Photoshop Professionals and I have to say I haven't seen anything remotely close to this quality. It is an amazing process and the images have to be seen to be believed.
The print quality is world class. Silky shadows, powerful highlights and depth beyond comprehension. Check out MPIX. They will truly blow you away!