I've used Katun toner for the CF2002/3102, C350/C450 and worked pretty well - a bit of deposits on the fuser fingers but I clean them. As for the C250 I only used it once - and then changed back to OEM

- the images were much darker, the text much blurrier and I think it messed up the developer as well... The C250 Katun cartridges come chipped.
As for the C250 cartridges thrown away still full - you shouldn't change the cartridge until the machine stops printing and asks for another cartridge - do not change them when the "x color toner needs to be replaced soon" message appears. The C350 cartridges do not have a chip, also the OEM ones have a transparent window through which you can see the toner - the Katun cartridges are entirely transparent so you can see exactly how much toner is left.