I refill my carts and I've experienced very little problems and very few failures.
I use a size 6 crochet hook to pop the end off. This hook fits inside the hole and has a notch to hook onto the inside lip of the cap. It pops off very easily and does not seem to damage the cap like using a thumbtack did.
I fill my carts at night and let them sit while I sleep. When I first started refilling them, I would overfill them and end up with juice on my tongue during the inhale or I'd get no vapor at all. If I let those sit longer or used a paper towel to remove some of the juice, they'd work just fine.
Now when I fill them, I use about five or six drops and let it sit while I sleep. I suspect I could add another five to six drops when I wake, but I've not tried that yet. I suspect this because a refilled cart lasts a fraction of the time as a factory filled cart. This must mean their refilling process involves more juice and it's probably added slowly. In two months, I've purchased 55 carts. (I thought I could only refill them once or twice.) I've only had seven carts fail. When they fail, set them aside and try them a few days later. If they still fail, I pull out the stuffing and put the damp stuffing into a syringe. I use the syringe to squeeze the remaining juice out of the stuffing. Johnson Creek has a great syringe for this.
I have not tried to boil, soak, or otherwise clean the carts. I just refill them. I haven't really switched flavors a lot so refilling them hasn't messed with the taste for me.
I am going to try to clean my failed carts, but I've not figured out how to do that. I've read about boiling them, soaking them in hot water, soaking them in rubbing alcohol, soaking them in vodka, and holding them under running water. I can understand any of those cleaning them, but my concern is drying. It does not seem as if one can pull out the stuffing without ruining the cart. I do not want to risk inhaling rubbing alcohol. >.< So, I'm thinking about getting a tub of DampRid at Home Depot and seeing if that will dry them. If I have the extra cash, I'll look into an electronics dryer (like the ones for hearing aids) and see if those work.
I recently ordered a specialty flavor and I'm on the look out for blank carts for it. If I can figure out how to clean the carts I already have, then I won't need to worry about finding blanks.
When it comes to actually refilling the carts, this is my process in detail (for those who want more detail):
Line up carts to be filled
Pop caps off
Place 2 or 3 drops in each cart by dripping the juice on the side of the cart along the stuffing
Wait and let juice soak in
Place 2 or 3 more drops in each cart
Wait
Survey carts: stuffing should be damp, but not soaked. Add more juice if needed. Do nothing except hope if a cart seems overfilled
Replace caps
Put carts in box
Place boxes of carts on their side in drawer
Once I try cleaning and drying the carts, I'll post my results.