A couple of years ago cartotanks were all that I used. I had at least 20 of them, each containing a different flavor. I was quite happy with them, although not always so happy with the quality control of the cartos that go in them. I had used Boge, Smoktech, and Ikenvape -- I preferred the Ikenvape Platinum.
I worked for a vape shop in 2014. I was the only employee who used cartotanks; the other employees hardly knew anything about them so they never sold any. My first couple of months I sold thousands of dollars in cartotanks according to my then store manager. After a couple of months there was a change in store management. I asked the new manager for some cartotank tools (carto punch tools and cartotank refilling tools) which I believed were essential for cartotank users and would help promote the sales of cartotanks in the store. Alas, new management wanted to take the store in a different direction -- rebuildables, as that was what was the new hot sellers.
Anyway, long story short, I began rebuilding myself as it was a requirement for my sales position. I stopped using cartotanks a year ago when setting up my last tank I got 5 bad cartos in a row. Personally, I'd had enough of the lack of carto quality.
I now use a rebuildable tank and drip atomizers exclusively. I don't have to depend upon the quality control of an underpaid Chinese factory worker. If my wick & coil build sucks, I have no one to blame but myself. It's cheaper, easier, and faster to rebuild than to use a cartotank. And the vape is argueably better, too.
There are still plenty of people who use cartotanks on this forum, although they are probably a dying breed. It's a shame, though. Many vape shops don't carry cartomizers any more. Some online venues are also discontinuing them. The carto & tank sub-forums here on ECF are almost dead. Some might say that people who learn how to use cartotanks correctly don't post much because they never have problems.
Baditude's Cartotank Setup Guide
IBTanked Pyrex Cartotanks - Review