I normally play on the NA server, have over 11k battles there and over a 72% WR. Here is my opinion on the +1/-1 MM.
It greatly improved the mid tiers (5-8). By that I mean it turned those tiers into easy mode. I had no difficulty having 30+ win streaks in tanks like the SU-100Y 100% solo, because it was ridiculously easy to seal club the low tier tanks when you don't ever have to fire normal AP, because the 530 alpha damage AP with "only" 170ish penetration was plenty to go through any tank that it ever saw. The same thing happened with tanks like the T29, KV-3, KV-2 (my god the KV-2 was OP), SU-152, KV-4, T34, etc. They became instantly OP, because the +2 tanks that kept them in check so they don't just go around roflstomping everything they see were no longer there. Tiers 5 and 6 are the only tiers that really benefitted from the +1/-1 MM, because they were no longer being shafted with +2/-1 mm. The tiers 5 and 6 not being shafted anymore, along with there being so many extremely OP tanks under +1/-1 mm made the mid tiers incredibly easy. Potatoes were still making the same stupid mistakes that potatoes always do, but there weren't any tanks that were higher tiers there to keep them honest, and punish them when they made a dumb move. This degraded the gameplay to the point that tanks at all tiers would just roll forward until they saw the enemy, and just sit there and fire. More often than not, they would trade shots in the open, because there weren't any tanks to keep them in check if they were in the seal clubbing tanks.
Now the higher tiers, however (9-10) were hit hard with the +1/-1 MM. Again, People were making the same stupid mistakes that people will do because most of the playerbase doesn't really care if they're any good or not. But in the higher tiers, because there are more T9s and T10s in the matches (because they cannot see T7 and 8 respectively), they were getting punished more for their mistakes, and they still would never learn. They would still roll around a corner sideways on when 3-4 enemies were aiming at them, just to maybe put a shell into an enemy tank. I had a game in my Leopard 1, where 2 of my T10 allies were dead 50 seconds into the match, and to make it worse, not a single enemy had been spotted yet. In a T10 game, under +1/-1 mm, every tank had to pull his/her weight in order to have a chance at a win. It was a 50/50 coin toss on whether or not you were gonna win. I had matches where myself and another SU platoon mate happened to get on the same team as another good player (around 60% or so) and we still managed to lose, because despite doing around 2-3k damage each, the rest of our team did what they always do (i.e. drive forward. see enemy. drive into open. die), but they were doing it at an even faster rate than normal. I would often find myself in a 1v6 2 and a half minutes into the game, not knowing wth happened to my team.
Now on the issue of premium tanks. Tanks like the T34, Lowe, SU-100Y (premium tanks w/o premium MM) would get an instant buff, because they had guns that were easily capable of dealing with tanks that were 2 tiers above them, because that's how they were balanced, so those tanks excelled in the +1/-1 (The IS-3D was also a monster in the +1/-1, 2400 DPM during the first minute of an engagement at T8 is crazy). However, tanks like the IS-6, AT 15A and P/M10 (premium tanks with premium mm) got an instant nerf. They were no longer seeing 1/4 tanks -2, 1/4 tanks -1, 1/4 tanks equal tier, and 1/4 tanks +1 (the tanks that they would struggle against). Now they (obviously) saw 1/3 -1, 1/3 equal and 1/3 +1 (again, the tanks they struggled against). The credit earning potential of these tanks was nerfed, because they would be forced to fire more APCR at the tanks that are +1, because their guns can barely handle them. An IS-6 would see nearly 10% more E75s, T95s, ST-1s, M103s etc, and be forced to sling more and more APCR because of them. Nevermind the fact that every single one of those tanks also doesn't have any problem at all penetrating the IS-6, which is actually another huge nerf to it, but the IS-6 would be forced to shoot APCR even while aiming at weakspots much more often, and the credit earning of those tanks, well... tanked.
The rational players on the NA server (not the ones who think +1/-1 is 100% better, or the ones that think +2/-2 is 100% better) think that there should just be a different tier getting shafted other than T5 and T6.
The mm should be
1-7 +1/-1
8 +2/-1
9 +1/-1
10 +0/-2
that way, only 1 tier is getting the short end of the stick (T8), The premium MM tanks can keep being special, and T10 isn't so much of a coin toss where even 3 SUs would lose to a team of 50%ers because we would get 4 people potatoing out instantly.
The arguments for +1/-1 have devolved into an "I play the mid tiers more, and so do the majority of the players, so we don't care if T10 sucks. We want +1/-1" and "T10 is the tier that I play the most, and it's the end game so it's the only one that really matters, so we don't care if the seal clubbing tiers suck. We want +2/-2"