I don't presume to tell people what they should or shouldn't do with their money, but for what it is worth I do have reasons for my choice to stick with OEM inks despite their higher cost.

First, OEM inks have been specifically formulated to work with that manufacturer's printers. The formula takes into account nozzle sizes, amount of cleaning agent needed to keep nozzles clear, etc. Third party inks do not necessarily fine tune their products for proper viscosity, etc. across brands. Now while there a number of photographers who claim no problems from compatible inks over long periods of time, there are also a large number on this and other forums reporting hardware problems which are tied to using the replacement inks. Enough so as to cast some doubt as to the risk such inks might entail. I have been unable to find similar numbers of complaints from those using OEM inks. Since my printers have expensive replaceable print heads, even one failure would completely nullify a year's worth of savings I might get from the third party inks. Until there are clear studies which indicate that such reliability issue are not a concern, I simply won't take the leap of faith and risk possible damage.

Second, most carefully done comparative studies that i have been able to find seem to conclude that, in general, OEM inks produce better image quality and more fade resistant prints. Sometimes the third party inks have done comparable jobs concerning image quality, but often that equality is limited to a specific brand or model of printer, and does not necessarily transfer across to other brand printers or models. In almost all instances, OEM prints have been found to have longer archival life, being more resistant to both UV and ozone related fading. My needs are for the highest image quality and archival characteristics that I can achieve given my printers, and from my research OEM inks better provide this.

Third, I make use of ICC profiles religiously. Using these profiles allows me to get consistent color reproduction which matches my monitor across my printers and paper choices. Most paper manufacturers do provide ICC profiles specifically built for use with a specific printer, using their paper and OEM inks. I have only come across one third party ink manufacturer who produces printer profiles for their ink (ConeColor - but only for Epson printers). While there are some who claim that they can get spot on color consistency from an uncalibrated monitor and across different printers and papers without ICC profiles, personally I am at a loss as to how to do that with the consistency I demand. I could invest in the hardware to create my own ICC profiles, or pay to have them made for me (about $25 a shot), but that would pretty much negate the savings. So, if you have a need for consistency across printers and papers, sticking with OEM inks seems a better way to go, IMHO.

In summary, IF you feel that the lower cost of the third party inks justify 1) the chance of risk damage and/or possible replacement of your printer, 2) documented losses in print quality and (in almost all instances) fade resistance, and 3) you do not feel it is important to use ICC profiling as part of your color management routine, then maybe third party inks will fit your bill. If the savings is outweighed by any of the reasons I've stated, then maybe sticking with OEM inks should be your choice.