After havin played with A68, F66, and E68 a bit more, I'll try to write down my thoughts, which will not be easy for the A68, so I'll start with that.
Kalkül A68
I do like colorful strings. I like when not every racket looks more or less the same. I like the performace of this string. Power is there. A lot and easy to get. Control is there, but that's more up to the player anyway, in my opinion. The feel of the string is what really confuses me. I tried it on the P4 first and it feels like it's a very elastic string, where the shuttle is pushed into the stringbed deeply. The weird part is that it doesn't seem to stay there much longer, even in the comparison to BG80 (where I feel the shuttle just barely touches the strin), so it does not result in a shuttle holding feel, that I really dislike. It felt like it lost quite a lot of tension at the beginning, but then stayed there and kept the feeling. If you like this kind of feeling of strings, there's nothing I can complain about. I'm trying it again on a P8 now to see if that changes anything for me, but if it doesn't, there's better strings for me.
Überschall F66
It's the thinnest string from Adidas and one that surprised me in more than one way. It stretched quite a bit on the machine, so I expected a softer feel. It felt super smooth, so I expected to feel the shuttle slipping on the strings, like I did with BG66UM when I tried that. I got a medium to medium hard feeling string and never felt the shuttle slipping. So much about my expectations. I was really surprised that this string barely showed any signs of wear and didn't lose much (if any) of the initial feel for a loong time. While it broke just after showing small notches at the intersections, I am happy with the durability for a string of this thickness, especially considering that it kept its playability over the entire lifespan. P68 wasn't able to do that. A68 felt like it lost much more tension. Very different from BG66UM which doesn't stay playable for long and snapped (due to fraying) much quicker when I tried it. I was able to switch between this and G-Tone 5 quite easily, despite one being quite rough and one being totally smooth. While GT5 looses playability over time, frays a lot, but takes ages to really snap (if you don't cut it), F66 stayed playable until the end. I'm seriously considering buying a reel of this! I'm also curious what LN1 users think, so I'm happy that
@s_mair is trying this now as well.
Spieler E68
It has much in common with P68, but while P68 didn't impress me and lost the feeling/playability rather quickly, I like this one. It feels nice off the machine, then looses some of the feel, but stays like that for quite some time. At a cheaper price than P68. I have said it before, but I'll say it again, it really reminds me of NBG99 performance wise. One of the kids I coach always tried new strings (BG65, BG65Ti, BG80P, NBG95, GT9, ...), but stayed with this one for some time now. She even recommended it to her doubles partner, so.... while I think I still prefer other string, I think this is way underrated AND under-promoted. Adidas marketing, don't promote this a a "cheap durability string", it's much better than that! My opinion, of course. BTW, I strung one of my P8s with it again, just to see if I'm missing something negative. While the other Adidas strings are made in Japan, this one is made in Taiwan. Does that point to the OEM for anybody?