It's mostly in the paper, since the same ink works well on their other papers. Best papers (today) are Epson "Matte Paper Heavyweight" (cheap, too!), and their new semigloss "ColorLife Photo Paper" (more detail, much more expensive).

But the PGPP is usually okay for most people. I still have some here that I reserve for scenes with a lot of tonal detail, like skies. I'm not buying any more of it, though.

Cheers