I began rebuilding because certain juices would kill my stock coils too quickly or every now and again, I would get dud coils (this was 5 odd years ago, coils were a lot more inconsistent) and I have never looked back.
It took me a long time to find my ideal tanks but I've got there and I am glad I don't have to rely on stock coils because it can really add up if you're replacing them constantly. A reel of wire and a bag of cotton last ages and will make a lot of coils for a rebuildable for a fraction of the price of your average pack of stock coils.
Things you need to pay attention with juices especially if you're using them in stock coils are:
- PG/VG ratio, if the juice if too thick, your coil won't saturate properly and will just gunk up/burn, too thin and it will just leak constantly.
- Additives, if your using a juice that is heavy on cooling (fruit on ice flavours etc) or uses a lot of sweeteners (dessert and candy flavours etc), the higher the concentration of additives like this are in your juice, the more likely it will gunk up and kill your coil pretty quickly.
There is probably more but those are the important ones that come to mind, what are you using at the moment? What's your device/devices of choice and what juices are you getting?