Why refilled and remanufactured ink cartridges are risky
Inkjet cartridges are designed to be disposable. They are not meant to be refilled. Most contain a chip which measures the amount of ink remaining. When the cartridge is empty, the chip is set to empty. Simply putting more ink in the cartridge will not solve this. The chip also has to be reprogrammed to a full state. The chips were not designed to be reprogrammed, therefore, sometimes this procedure is successful and sometimes it isn't. I have seen numerous complaints on the net of people who bought refilled or remanufactured cartridges only to have the printer report that they were empty immediately upon insertion.

How does the chip know when the cartridge is empty? There is a floating sensor inside the cartridge. As the ink empties, it nears the bottom of the cartridge and eventually reports it to be out of ink (often when there is still 10-20% of the ink left). Once the chip is set to empty, it will not be reset by filling the cartridge with ink. Even though the sensor is once again floating, the chip still has to be reprogrammed. But what happens if the sensor becomes glued to the bottom of the cartridge by dried ink? Even refilling the cartridge and reprogramming the chip won't help because the sensor will report that it is empty.

Inkjet printers squirt ink through microscopic holes. If these holes get clogged, it can cause problems from poor print quality to complete printer failure. However, all ink has a tendency to coagulate when exposed to air. Otherwise it would never dry. For this reason, you should turn on your printer once a day even if you don't print. It will send just enough ink to keep the heads from getting clogged. (This information was actually given to my sister by a Canon technician when her brand new Canon needed a second head replacement within three months and she had been using only genuine Canon ink.) Now think about an empty ink cartridge which sat around for a while before someone injected more ink. The remaining 10% of the ink in that cartridge has coagulated. Now it has been refilled. You are already losing 10% due to the coagulated ink which was there at the time of refill. But the problem is even more serious. If that old ink clot breaks free, it can clog and permanently damage your printer.

Note that refilling a used cartridge is not the same as using a continuous ink system. In a CIS, the ink continues to flow and therefore it solves several problems. The sensor never reaches the bottom of the cartridge to report that it is empty. The chip is made to continually refresh itself so it never reports empty (unless it does actually run out of ink). The ink is free-flowing and not exposed to air, so it doesn't clot in the cartridge. While I don't personally use a CIS, since compatible cartridges are cheap enough and more convenient, for someone who does heavy-duty color printing and prefers an inkjet to a laser, it seems to be a good solution.