I tried my friends fogger v4 but its a bit like kayfun. But the air flow is really nice. I guess I'm a bit picky when it comes to rbas
Kayfun was my first RTA and I loved it, but once I started using drippers and other types of rebuildables I REALLY started to like the open type airflows, and the heavy vapor production. Honestly, IMHO, I don't think Kayfun's are up there at all in terms of vapor production or airflow. Even with the screw all the way out its still like a thick Mcdonalds milk shake lol. BUT..there is one clone on Ebay where they modified the center pin, and opened up the air flow so its actually VERY decent. Its called XXL Kayfun. The threading is one of the best I have seen on any Kayfun. VERY SMOOTH.
Kayfuns how they stand I think are optimal for mouth to lung hits. If your a direct lunger who likes open air flow, I wouldn't bother (unless you went with the XXL). Out of the Aqua, and Various Foggers, the V4 Fogger has been a beast for me, never leaks (after I learned the right way to build it. All tanks require slightly different tweaks to work best) and makes the most vapor out of any dripper or tank I have ever used, and blows away anything even in terms of flavor. So I am another one who is gonna suggest the fogger v4. Its got the pyrex glass, and is just a beast in every way.