I would like to make a few remarks about the refilling ink market.
I work at HP in a very large R&D location for Imaging products (Printers, scanner, etc.). HP has put in over $2mil in research to make sure their products work as well as they should. Everything from a sound proof room (to make sure the sounds are not annoying, but you still know that the printer is working), to environment chambers where we can simulate conditions in any part of the world. (do you live in Saudi Arabia where there is a ton of sand? We got you covered. Vietnam's humid climate? Good to go.)
One thing that HP hates (and you can take this with a grain of salt since their biggest price point is supplies) is when people re-fill the cartridges. You see the devices are engineered to work with their paper. They are engineered to work with their ink. They are tested in different sorts of environments to make sure that the combination of ink (or toner), paper, and device all work optimally.
Now a little about ink cartridges:
There is a bag inside of the hard cartridge that you do not see. In order for the ink to come out of the cartridge the bag is pressurized to cause the ink to come out of a VERY fine hole at the bottom. When you re-fill the cartridge what you are doing is puncturing the bag (no matter where you put the hole) to get the ink in. What this ends up doing is causing a weak spot in the bag when the bag is pressurized.
It may never happen to you for your entire life, or it may, but one day the pressure will be too much for that bag and it will explode all over the inside of your printer. This ink will gum up the inner workings of the printer and you will be faced with a repair job that costs the same, if not more, than a new printer.
Again, if you have been doing this very thing and nothing like this has gone wrong, then you are one of the lucky ones, but beware it could happen at any time.
Also, tskipbarry, unless you are printing pictures, you might want to look into laserjet printers. They use toner which is a much faster process of printing, are much cheaper in the long run, and give you the same quality of documents. If you are printing pictures, then I can understand the need for the ink as you will not find a better quality in toner.
Whether you heed my warning or not, I hope this information helps you make the best decision for you. I am in no way trying to tell you to re-fill or not, just the engineering side of it.