Welcome to the forum, there is a wealth of user knowledge available here!!
I started with the V2 kit 4 months ago and have tried several other KR808D-1 cartos and found that the V2's held more
juice and lasted for more refills than the others I tried. I am not saying that this might be your experience as I did not try many others. I have moved on to other devices now for my main vapes but still use my V2 where the larger ones would be too cumbersome.
Also the prefilled cartos can be refilled as many times as a blank, I have refilled several cartos over 20 times using a clear juice such as menthol making the actual cost of the carto per refill less than 18 cents.