The RTA (Re-buildable Tank Atomizer) basically, allows one to make their own coils. By varying the type of wire used (Kanthal, Nichrome, Nickle, Titanium), the size of wire, and the type of wicking material. It can in the long run be cheaper than buying the pre-made coils. Or allow one to make a setup that you like the best.
You can keep rebuilding as long as the hardware will allow it. If you take care of the hardware (upper, middle, lower) by not striping the threading of the parts (upper to middle, set screws) it should last for quite a while. They do provide extra O-rings and insulator rings incase they wear out or get damaged. Those parts should be able to be purchased in the future, if needed.
Right now I am using a build that is 28 gauge kanthal that was wrapped around 2.5 mm shaft that ended at 0.75 ohm, built as a BVC. It is nice compromise to the 1.8 ohm and the 0.5 ohm Atlantis coils. I can close down the lower airflow and lower the wattage to make it nice mouth to lung hit. Or if I feel like it, open up the lower and upper airflow and increase the wattage for a solid lung hit.