What you save by not buying prebuilt coils, you'll end up spending on more juice. The subtanks are notorious for sucking down juice like nobody's business. So, don't go into it with the thought that you're doing it to avoid paying to buy pre-built coils by rolling your own. My juice use went up tremendously as did my partner, but the saving grace is that we don't have to replace the OOC coils every couple days. I honestly believe it's about evened out for us, coils for juice costs.
I do love the tank though and honestly can't find anything else that I'd want to replace it with, for me it's nearly perfect, plenty of vapor, flavor and airflow and for some reason mine doesn't leak like some have reported theirs doing.
All I'd need from it to call it absolutely perfect would be a cleverly designed top fill port.