There are some risks involved with refilling cartridges. You must be careful when refilling. The cartridge must be resealed after refilling. If it leaks into the printer, it will probably ruin it. If it does, your out a printer and haven't saved any money. Don't let anyone fool you, refilling cartridges can be messy too. Be prepared for that. For instance don't refill them on your nice oak kitchen table. I know a guy who ruined a couch.
The other thing would be ink quality. Don't go for the cheapest you can find. The ink nozzles on the bottom of ink cartridges are very small (thinner than a human hair). It doesn't take an impurity of any big size to clog the nozzles.
Lastly, minor things in the printer driver may no longer work correctly. Most HP printers track ink usage. This will no longer be accurate.
If you are only buying a few cartridges a year, it probably isn't worth the hassle to refil the cartridges, but thats just my opinion.