I tested M and M+ on a ZJK-ALC blade. Because I bought the rubber myself for market price, and have no business connection to Nexy, you can consider this an unbiased review.
For the last decade, I have played more or less exclusively with ALC blades and either Tenergy 05, or most recently, MX-P. I am a conventional two wing shakehand offensive player, getting older now, but still around 2050-2100. These days I have worried that I have had a tendency to beat myself more than I would like, missing shots from mistiming -- a lot of that secondary to not being as quick as I used to be (and maybe needing new glasses).
Here is what I have noticed so far with these rubbers (some of which echos what Next Level wrote on his earlier review):
There is a slight dome of the rubber right out of the packages (true for both M and M+). The sponges are blue color with very thin pores. There is no booster smell, in marked contrast to MX-P. I had forgotten how much easier it was to glue down and cut rubbers with thin-pore sponges like this one. A very nice feature.
The pips on the inside of the topsheet are incredibly short (I've never seen anything like it before) and they have a shape that is somewhere between a circle and a hexagon. The topsheet therefore, overall, is very thin. I think this may be the key feature of this rubber. (I hope it does not make the rubber fragile).
These rubbers are not real fast, and not unusually spinny (although they certainly spinny enough and the topsheet has a very nice grippy feel). The main thing is that they are incredibly predictable, which became clear to me within a few minutes. That is the thing Nexy talked about in his threads (at MyTT, use the search function there) on what he was trying to achieve when he developed the rubber. I have to admit I was very skeptical about that when I read it. It sounded like BS to me. But having tried Karis, I now think he has accomplished what he set out to do, and his descriptions ring very true once you try this stuff. I'm not just talking about his descriptions of Karis, but also how Kris is different from Tenergy and ESN rubbers. When you play with Karis, you sort of feel what he is talking about. They really are something novel I think.
More than anything than, I would say the main features of Karis are predictablity and linearity. (Output is directly related to input with no weird discontinuities to put it in more quasi-technical terms).
Because of the predictability of the rubber, I was able to keep the ball on the table really well tonight. When I wanted to hit slow, the ball went slow, and it went where I aimed it. When I swung harder, the ball went faster, and importantly, no sudden increase in ball rebound with a slight increase in racket speed. (That is in marked contrast to the MX-P). So very few inexplicable misses where you hit the ball off the table and you're not sure why.
One of the places this really showed up is against my opponent's good shots. I brought a lot more of them back and lived to battle longer in the point. I think most amateur offensive players will find that they are a lot better when they are on the defensive with this stuff than they would be with a lot of popular rubbers. In general I kept the ball on the table, but I also felt like I was able to be a bit more deceptive as to where I was going to place the ball. In other words, make it look to opponent like I am going to drive the ball crosscourt with my forehand but then go to the body or down the line at the last minute. For some reason, that was easy with this stuff. Also I was hitting really good angles and I was looping with a lot more variation in pace.
For now the only downside, and I think it is temporary, is the sense that some of my shots lacked the penetration I would get with Tenergy or MX-P. That's to be expected, Karis is definitely slower than either of those. Also, it seemed like I was hitting my forehand flatter than usual, probably subconsciously trying for more power to compensate for speed of rubber. I guess I would say that the throw is medium, though. I will need to play more with an eye on that more. In any case, when I got good body rotation that was not a problem, then the ball had plenty of pace. It is better to keep the ball on the table and win more points than it is to look spectacular winning a few points, so I don't mind. This sense of very linear control was present on every single shot. Loops, counters, blocks, short game, serve, return, push. I will need to work hard to get good body rotation on my opening loops in particular, because this stuff definitely does not reward lazy technique. You don't need to be Ma Long b ut you can't just loop with your arm. If you do, it will feel like it is about to fall off in a couple of minutes and your shots will suck!!
As for M vs. M+, given what I have been playing with for the last decade (T05 and MX-P), M+ is a step too far, especially away from the table. I definitely preferred M. Actually, for now I am more comfortable with M on the FH side and M+ on the BH side (which is a bit odd since M+ is distinctly harder), but I plan to use M on both sides for awhile and see how this goes. I ordered another sheet of M today so I will have it on both sides.
EDIT ADDED AFTER FOUR WEEKS OF PLAY WITH KARIS M BOTH SIDES.
I am still using Karis M on both sides and would not change much about my review. I can now say that durability is about average for modern rubbers. I don't plan to change any time soon.
However, one problem has emerged. Several of us have noticed that the sheets have a lot of weight variability. I bought two new sheets to put on a different blade (my favorite Viscaria). The Black 2.2 weighed 67 g uncut, while the Red 2.2 weighed 61. The sheets were the same size. That is way too much weight variability for a $50 rubber! Andy Smith noted at MyTT that he felt some really hardness variability from one sheet of M to the next. Nexy has to do better with their quality control. Fortunately, I still liked it fine, settled with the Black sheet on my FH. But not everybody will be so forgiving.
EDIT added in late September of 2017. I finally replaced two sheets of Karis M on my Viscaria that I put on in Late January of this year. That is nine months of play, three times per week on average. I would not change a word of the review, except that the durability is beyond belief and I like more than ever.