9 of them in a large 3-story school building (2 on each floor). The building is in a Γ shape (35 meters each side) and covers the entire space. One is a wired master and the rest are in mesh. They operate in "Access Point mode" and all management is done through a router via vlan, so I have no opinion on the integrated functions such as DHCP, QoS, etc. The guest network is convenient, just open/close it when needed, but it doesn't have any other settings in "Access Point mode". With this setup and a good router, I have seen up to 80 devices connected without any problems and it never drops. Due to its shape and minimal weight, it is very easy to place them high on walls or even behind glass for better performance and coverage. They have a functional range of about 20 meters through a wall. In outdoor areas, the range does not go beyond 25 meters. The downside is the cost, which I find excessive, especially for home use where there are much cheaper solutions available. For large spaces and multiple connections (30-40 and above), they are worth it.
+ Stability
+ Multiple connections
~ Range (moderate)
- Cost

2 sets of 3 were purchased. Total of 6 pieces. Version V 3.2. The machines play flawlessly, they have stability without disconnections and integrated network. That means you don't need to enter a code and reconnect every time you go to a different room. It makes your network unified, with no relation to repeaters that have a different way of functioning. You connect the first one to the central router and the rest as relays. It also has the option to prioritize certain devices and the option to build a guest network so that they don't connect to your home network. In a 70 sqm house, it covers the whole area with excellent networking. If you have complexity in your house with walls and obstacles, the set of 3 is just enough. Of course, they are also sold individually (in my opinion, it's not cost-effective). It's better to get the set of 3. I use one of the 6 only for cameras and another one prioritizes two other wireless cameras and the recorder. Truly one of the best machines that solves all kinds of network problems. EXPERIMENT A BIT WITH THE PLACEMENT OF THE DECOS TO GET 100% OF THEIR CAPABILITIES AND RANGE. FOR EXAMPLE, IF IN ONE MACHINE (DECO) IN YOUR APPLICATION IT SHOWS 2 GREEN WIFI INDICATORS, BY MOVING IT, IT CAN MAKE IT SHOW 3 INDICATORS, WHICH MEANS PERFECT WIFI SIGNAL RECEPTION

It was purchased and installed yesterday, 15/1/2022, replacing a Netgear Nighthawk R7000 at my parents' house.
Due to the layout of the house in a "P" shape around the elevator shaft, the installation of only one router had to be done in a central location of the house, so it was placed on a high shelf in the hallway to be approximately in the middle of the building. In the same position, other routers simply couldn't reach the signal throughout the house. Due to the upgrade to broadband and the phone becoming VoIP, the router had to find another position. Instead, I decided to try the Wi-Fi Mesh with the set of three Deco M5. The installation is really easy, and the fact that they can be controlled remotely from the mobile phone is incredibly convenient for me, as if my parents have a problem, I can check it without having to go there, and ultimately, as promised, it fully covers the space without any issues.
Specifically, I have a very strong signal in all rooms and corners of the house (which, I repeat, may only be 120 sq.m. but due to its configuration, it makes the Wi-Fi signal difficult enough), while in many rooms, I can locate 2 or even all 3 Decos with my mobile phone! This means that even just one Deco has a good range and strong signal to cover a challenging area. (It wouldn't easily cover the entire house on its own, but I would definitely get the job done with two)

The product is the best you can get for this money as it does exactly what it says.
The review is after about 2 years of use and I can say that I did not expect it to be so stable and reliable after 2 years. I have tried in the past all other ways to cover a maisonette of 120sqm and I always had problems. Initially with a simple repeater and later with powerline, and in both cases the coverage was not so good and when there were more than 2 wifi access points using powerline, after a while it would freeze and not switch automatically from one access point to another, which was particularly annoying!! After installing the mesh, you simply forget about all that. No restarts, no freezes, no drops in speed!
Also, the installation is very easy through the application from which you control the entire setup with great ease. And all 3 are wired to have the maximum speed everywhere, even wired, as there is an inlan connection (200 up / 200 down) and a NAS server on the network, so I can download from the server to a device via wifi at over 30-40mb/sec. TP-Link did not disappoint me this time as many of the previous setups were also with TP-Link devices and there were many problems.

It concerns version V3 with the two devices.
I bought the product for €99 with the intention of adding it to a two-story, 150sqm house.
In the space, I have two M4R devices that work perfectly. One of the two M4R devices is CONNECTED in ROUTER MODE with PPPoE, and the second one acts as a repeater for the first one. The Mesh network operates normally with speeds of 866 on 5GHz. In the 2.4GHz range, I only have smart devices that connect without any issues. I assumed that the M5 devices would perform better in terms of speed and range because they have more antennas for 2.4GHz. Additionally, the M5 devices have anti-virus and an integrated speed tester. Unfortunately, replacing the M4R devices with the M5 devices did not yield better results in terms of speed and range. Furthermore, the Mesh network does not work as well as it did with the M4R devices. Perhaps the additional 2.4GHz antenna is affecting the Mesh network on 5GHz, and the devices are holding onto 2.4GHz as the stronger signal, but this significantly reduces the speed. I should note that the two M5 devices were placed in the same positions and with the same settings. If you search the TPLINK forum, you will find some references to the problem with the M5 devices and the inability to automatically switch devices from 2.4GHz to 5GHz when a strong signal is available. For now, I have removed them and I am waiting for a firmware upgrade to see if it will be fixed.

3rd evaluation (after ~1 month of use)
I would now say that the product is worth **** (4)
I subtract 1 due to the high cost of acquisition.
Functionally, it is good.
The application works well.
Personally, stability came when I used them as Access Points.
At home, I have structured wiring with 2-3 ethernet outlets per room.
The difficult part was having stable WiFi because it is a stone house with incredible insulation and a strange layout (3 floors).
Ultimately, I use each unit via ethernet (so the package should have 3 cables instead of just one).
With this arrangement, the network is stable.
I have added several tplink WiFi bulbs and they all gradually appear.

2nd evaluation after ~2 weeks of use and while I bought a second set to better cover the spaces. Now I have a setup:
Basement 1
Ground floor 4
Attic 1

So I was able to completely eliminate the dlink ones.

I initially wrote that the package only contained one network cable per set, but this was my misunderstanding of how it works. In reality, only one needs to be connected to the network and it creates its own WiFi network that provides WiFi and LAN to devices. It has other modes of operation as well. So it can replace the structured wiring.

There are some instabilities that cannot be explained.
At some point, one deco may "lose" connection. Reinstallation takes about 10 minutes.

I give it 5 stars because I can't give it 4.5 stars since the price of €65/piece is a bit steep, but I also don't want to unfairly judge the product.

The ease of installation is proverbial.
With the evolution of Android, we have now entered the era of plug & play for networking products and it is ENJOYABLE.
Previously, it took me a lot of steps to set up an access point. Now it's... child's play... tab, tab, tab, done.
And the nice thing is that it works immediately and correctly. No more bugs, unexplained crashes...
It recognizes the type of connection, has two operating frequencies (2.4 & 5 GHz)
Gradually, it detects the different devices on the network, notifies for cameras, while with "smart" tp-link devices it provides full configuration capabilities, operation scenarios, etc. Flawless!!
The issue of range, which was a major criterion for selection, satisfies me initially and is definitely at least 50% better compared to two old dlink dap1360.
As a downside, I would mention the slightly high price.