The knockoff inks won't last as long, or don't have as wide of a gamut, or are more likely to clog the print head, or all of the above.
My experience with clone inks is nothing like Jonathan's. I've used clone inks for years with few problems. I have clone ink prints hanging (behind glass) in direct sunshine for over five years with zero fading. More than I can say for their Cibachrome friends nearby.
Ink gamut may or may not differ between manufacturers, but IMHO, it would take a forensic examination to reveal significant, consistent visible differences. These can probably be compensated for.
Clogging is an issue whatever the ink. The only difference with Epson inks is the fact that clearing the clogs costs you at least four times as much.
In my experience, If you're paying for the ink yourself, clone inks are good value.