What I like about a 21700 mod is that it can drive a larger tank (like a Cloud Beast) without the hassle of pairing 18650s. I started off with an iJoy Capo (crap) and an iStick Pico S, but recently splurged on an Asmodus Amighty and I love this mod. I've been using a small variety of 40a batteries; they discharge faster, but perform better than the 30T in my opinion.