Just come back from a good three hour session with VS-850 on my T2 and I was impressed. I used 27/30 on my overclocked crank (and put a Li Ning skunk tail on to decorate it) - here are some inital impressions:
850 strings very like a watered-down BG80. It's rough, but less so, and doesn't twist up as much. It does have that classic BG80 SHRIEK when you pull crosses, though, which takes me back.... It's a bit less stiff than BG80, giving me more on the prestretch and more movement of the crank head at the same tension than BG80.
I've never gone this tight with anything under .70 so I was unsure what to expect, but my very first hit (a big lift from chest height) pretty much set the standard for the rest of the afternoon.
The sheer feel of the stuff is unreal - when you hit the sweetspot you KNOW about it: it rewards you with a big, crunchy STAB of impulse, which is exactly what I look for first in a string. I'll push the boat out here and say this is possibly the best "feel" string I've ever tried (even better than my previous Z70 at 32).
Power isn't really a concern for me with respect to strings - if it hits hard enough I'll take it - but the 850 has enormous reserves even at 30 lbs. I was smacking the shuttle to every corner wth minimal effort and hitting a lot more winners from the back than usual. This, coupled with the musical high-tension THWACK, made it a real pleasure to unload on the shot.
The BG80-like surface gives excellent slice potential. I use some degree of slice in many of my shots, with driven service returns being heavily represented, as I slice over the top of them to keep them low. I tried a fair few of these today, and the excellent grip of the 850 really turned the shuttles and gave me some FLAT shots. The high tension was probably responsible, to some degree, for the precision of the day's net spinners, but I've certainly no complaints here either.
Lastly: tension loss. It's probably to early to be able to get a bead on this, but I didn't really notice anything substantial between the first game and the last; the string performed pretty much identically throughout the whole afternoon.
Based on today I would say VS850 is a winner. It remains to examine tension loss (and, ultimately, durability), but with respect to playability alone I can't fault it. It's not the most powerful string I've ever tried, but it definitely doesn't lack any power (and has the advantage of being reasonably thick) and for feel it's second only to Z62, which I now consider a freakish oddity so it doesn't count.