I just tried to photograph the difference, and you can't see it in photos. It's also hard because I don't have the same photo or page printed both ways, so you can't make a direct comparison. Scrapbook papers and elements don't seem different regardless of whether you do or don't use the 6-color. (Again, maybe if I had the same page printed both ways you might see a difference, but just looking at different layouts it seems fine both ways.) There is a small difference in the pictures, mostly in skin tone. I notice it most when the photos are imperfect and have shadowed areas on the skin--I feel like the 6 color printing gets those areas a little more accurate and less dark. But without looking at a book made with and without it side by side, I don't think you'd really notice the difference. If I handed you a book made with the 6-color printing and didn't tell you, you wouldn't be like, "Wow--what's different about this book? It looks great."
I think my take on it is that it's a nice add on if you're feeling like your book was a great deal and don't mind spending a little more. If you're feeling like your book was expensive, it's not worth spending more on it.