Hi WG,
I can speak with some authority on Laser Toner re-manufacture, as someone who once refilled his own LJ toner cartridges (back in the days of LJ II and III) and I know just how variable the process can be! I don't know much about inkjet replacement, and I've heard roughly equal amounts of good and bad for them.
The toner itself may be of a different spec. to the OEM variety, while the photosensitive transfer roller may be either worn, or a replacement part with different characteristics. Poor assembly conditions can expose the photosensitive roller to excessive light, which reduces its effectiveness (resulting in fainter printing).
Other problems are due to misalignment of the components in the re-assembly of the cartridge. It is also much more likely that a re-manufactured cartridge will leak, as the splitting open and re-sealing process results in a poorer seal than the OEM part.
I use HP branded long-life toner carts in my home printer, and always return the cartridges to HP (my bit for the environment).
However, it is not clear to me exactly what HP actually does with them (other than prevent them getting into the hands of re-manufacturers). I don't believe that HP could re-use them directly any better than anyone else, and my belief is that they don't even try! But they do say that they recycle them in some form, so...
SD