Can't say I have or even know anyone using it. My observations from watching OOAK forum indicate a lot of people in Australia still use all versions of Mark V though. So if you are patient enough, you could get a reply from there. That is a good value of a Table Tennis forum, you can ask away and find someone who knows so you get a valid opinion without spending money and failing.
I am so anti-establishment / anti-pundit that I stay away from their blindly recommended must-have stuff. Sriver / Mark V is part of their creed.
Unless I run into someone at a club (then I would ask to try it a quick hit) then you will not see me buying it to know.
Take Korea, which most of us view as a pretty well developed TT nation who knows what they are doing more than many other nations. We see Korea that way. I spent 4 years there playing daily 7x a week, doing club, city, regional and national tourneys. I visited a different club at least 2x-4x a month. I have played vs thousands of amature players there. I check their bats.
In the entire time I was there, I absolutely NEVER saw anyone with a sheet of Mark V. I mean NEVER saw the stuff. I ask players and they never heard of the stuff or know its history. (They use the shyt out of Yasaka Extend HS though - you see that all over the place) I saw exactly ONE sheet of Sriver in use there. ONE. SHEET. That is it.
I know your stance on this
but do give it a hit if you find someone using it. IMHO it's a great rubber. For a non-tacky non-chinese, non-tensor rubber it has great spin (while serving (though you have to work for it) and during the match. I twiddle sometimes while receiving (depends on the opponent) and in case the service drifts long, my opening loop with Mark V has enough spin to cause Kreisklasse A/B players to have trouble keeping the ball on the table. They are able to control opening loops from my FH rubber without too much trouble though.
I also sometimes twiddle to do a heavy backspin serve. Opponents are able to return the service if I use my FH rubber but if I twiddle for the next service the return hits the net quite often.
It does have its drawbacks. As it is a kind of do-it-all rubber, there will definitely be an XYZ rubber that can do something that your game requires better than Mark V. It's fast enough but it's not that fast. I think it's a tad overpriced because of the Yasaka Brand. 20 euros instead of 26-30 would have been a better price. The durability is a bonus though.
I have tried Sriver and IMHO it's slightly slower and definitely less spinny than Mark V though that could have been because of the blade.
For me, Mark V is an "almost" perfect rubber. It's great on FH but I feel that a slightly faster rubber could be better and it's fantastic on BH but I feel that it would be even better if it were a tad softer.
Advanced players aren't really using Mark V at my club either (Tenergy, other slower Butterfly stuff, Bluefire or Spinlord is the norm) but a good deal of them know about the rubber.