I've been refilling for about 12 years now and refilled my daughter's Kodak cartridges a couple of years ago. I think the main problem with any cartridge is finding the filling hole under the labelling and checking with a wooden cocktail stick or matchstick to find out which colour you need for that particular hole.
I would also advise refilling BEFORE they run out. If you get white or pale streaks in the printing, this means that the heaters are heating up without the cooling ink surrounding them and they soon burn out and are unrecoverable. I have a digital scale which I use to weigh the cartridges and consequently get a good indication of when they are 2/3 empty.
Most printers have a system of making refilling difficult for obvious reasons but there are ways round most of them.
The last HP cartridges I bought were refilled for two years until I removed them incorrectly and was forced by the inbuilt system to replace them. They are however now refilled and waiting to be used later.
Nice to know that someone else refills.
Hope this helps.
Wak.