Well, the v5, as so eloquently stated by Senor Bronze, was marketed as a “tank for all”....but did not turn out that way in practice.
To be honest, SM was really onto something with the design of the v4...by and large, the v4 was my favorite Kayfun, until the Lite19 came out.
One of the biggest controversies about the v4 was the reverse threading system for the juice control, which in my opinion, was amazing...but the general public essentially refused to learn how to use it, or accept it...and therefore SM returned to forward threading on the v5...
In my experience, with the two I picked up on launch day, the forward threading caused the tank to separate from the chimney every time I opened the juice flow. No matter what I did.
Keep in mind, I’d been using Kayfuns since the 3.1 came out, so I was no stranger to their ecosystem or rebuildable tanks in general.
All the above, along with the massive size, and the v5 ended up being the only Kayfun I ever sold.