Yeah, but if there is a local vendor that sells OG kayfun 5, they might have one in stock.
Very true. None of the usual US online stores (OriginVape / Dicodes-USA) have them anymore. I'm also with you, in Europe if there are any more OG K5's to be had they're likely to be found in some local shop. Creme de Vape, InTaste and StattQualm don't have any.
I don't own any of the other ones or the Prime so I can't say much about them. Was gonna buy a prime but I heard it was tighter than the 5 so I passed.
The OG K5 and K5 SE starts at 1.8mm AF. That's the same as wide open on a KFL+. I have a few authentic Primes. The airflow is stepped: 1mm, 1.2mm, 1.4mm, 1.6mm, 1.8mm and 2mm. I always have a Prime in rotation.
The reason I was attracted to the KF5 (clones - which I bought long after it was originally released with the exception of the one SXK clone I have) was for it having full metal sleeve, like the KF4. Yeah, not being able to see your juice level sucks, but I like the idea of not having glass to break. I've learned to live with it, but I do use it with glass as well. It's nice to have a tank that's built like a tank. lol
I have gone full metal on my K5 and Prime's. The Prime does take the K5 full metal (6ml's). I used them like that for awhile in both wattage and TC modes. At first I was leery of not being able to see the juice in wattage mode but learned it was no different than using an RDA. I could feel less and less vapor. So I knew it was time to refill.
I upgraded my Primes to the PEI kits which gives them a nice little 5mm window so I can see my juice. I prefer to see my juice levels.
The KFL19 and KFL21 or are also very nice MTL RTA's. I have the AF set wide open on my KFL19's (1.8mm) and for me it is a really nice airy vape.