Image quality (Score 1)

Using other people's ink can sacrifice image quality. Printer manufacturers spend a lot of time and money producing color profiles for their inks and papers. Using different inks will throw this off. You'll also have to find the best driver settings on your own. Third party ink probably won't be absorbed into the paper the same as HP's or Epson's. No one's inks are the same. There's also the matter of print heads gunking-up.

Just realize that you are giving up convenience and quality when you refill ink cartridges. Printer manufacturers must think that people don't realize this and have to force a decision for their own inks through weird licensing and warranty policies.