They're junk, don't use them.
This is especially the case with cartridges such as the ones used by most HP inkjets (vacum state inside). Refill a HP one and it'll print like crap, that's if the ink decides to stay in the cartridge at all. Another point, you can't refill the tri color ones.
As far as the cheaper replacements, most are junk too. Some cartridges have print heads built-in which have a short life span, generally as much as the original ink. Those cheap refills are recycled and haven't had the printheads replaced.
If you're wanting to save moeny, either print less or invest in a cheap laserjet. You can find some nice ones for around 100$, just set the quality to draft for stuff that isn't important.