I've refilled mine several times and have noticed no decrease in performance. I still get great
throat hits, flavor, and vapor. I've been using the prefilled black cartos with silver threads and hard end caps from litecigusa, but I received four boxes yesterday (not pre-filled) with soft end caps. I washed most of the hard caps about a week ago in hot water that I had brought to a boil and used a larger syringe to push the same very hot water through the filters, and they dried out fine overnight (with a fan blowing on them).
I was using the condom method of refilling, which has worked fine and is quick and easy, but a little messy. I just filled 10 of the soft caps using the 10 mL syringe I bought from Wal-Mart that comes with a little rubber white cone-shaped thing that the syringe goes into on one end and the carto hooks to the other end (threaded end of the carto). I was able to fill each carto quickly, with less mess, and watch from the other end of the carto (with the soft cap taken off) to know when the carto was full. Each carto took about 1 mL (or a little less or a little more) of liquid to fill.
There's a YouTube video of using the Wal-Mart syringe and the rubber cone-shaped thing, but I don't have the link right now. You should be able to refill cartos many times, and just clean them every week or two using very hot water and a syringe.