I starred out by filling my own blanks but
had to try some prefilled. The result? Unpleasant undertaste and fewer refills IMO.
Something about the juice that's been sitting in the carto for God knows how long makes it not only less palatable but harder to refill successfully.
Folks like doots and mwahle convinced me (it didn't take much) to just skip the prefilled and stick with blanks.(Thanks

)
Dripping/syringe along the wall of the carto (NOT into the wick, which I found damages the wick and shortens its life span)
+ taryn's spin trick
+ topoffs long before it can get burnt
= superlong flavorful life on a cartomizer.
Getting 5-10 refills per cartomizer, not as many as some, but I'm kind of using them hard on an HV model.