I'm waiting for the reviews of the masses on this new Kayfun Prime. I'm hoping that it's all it's promised to be. For real ...
I bought and vaped all the iterations of the Kayfun and still ended up back at the V2. I vaped the K3 for a really long time until I picked up my first KF2. I liked the Phillips fill screw on the bottom over the side Allen screw on the V3 and the slightly shorter height.
The K4 and the K5 lost me quickly. Flavor was slightly better, but the design was a little over the top (Lots of extra parts for the slight flavor increase). I still have three K4's and one K5.
V2 is just too easy and familiar for me. I don't need juice flow control or widely adjustable air flow. The big selling point is that it's a no-leak pocket carry. That's a major plus in its column.
That said, I really like the Kayfun Mini V3. It's a flavor tank and one that is pocket safe. It gets filled and vaped here and there, but the juice capacity makes it more of a home use tank.