@RandySea2 
I got mine set up yesterday. I was initially impressed with the set up ease........and then I noticed what everyone else has noticed.
Emperor's new clothes.
The ludicrous situation I have is this......I stand next to the Deco in my office and run a speed test. However my laptop is actually connected to a downstairs Deco much further away.
So I force the laptop to use the Deco in the office that I am standing next to and repeat the speed test. I get a slower speed!
This effectively makes the 3rd Deco in my set up useless - it is adding NOTHING as I am getting a better signal from a Deco much further away.

This is not a true mesh system. TP-Link are on very dodgy ground claiming it is. 

I will be returning it as it has not significantly improved the WiFi in my house that I was getting from an Apple Time Capsule, extending with an Airport Express,

Same issue here with a Deco M9 Plus tri-pack. This doesn't make sense at all? I've got 3 levels: living room ground floor, bedroom, attic. I've installed in order from bottom to top.  Now the attic has a dodgy connection to the ground-floor, and the bedroom is connecting to the attic????? This is all backwards....

Any suggestions how to solve this? Otherwise i need to return it i'm afraid. Forcing the connection source would solve the issue quickly.

@MackPhoenix62 Is ANYONE going to respond to this? JD Power Customer Satisfaction Award what???

i am having the same issue here....it like the deco m5  on 1st floor always connect to the deco m5  third floor ..and i do have deco m5 on my 2nd floor..it suppose connect on the 2nd floor since it the nearest one. just dont understand why it connect to the farthest one! .. tplink need to fix this..yes it frustrate when the math dont do it right.

I've also got the same problems with the M5 units not linking correctly.

Also would like the ability to set devices to only connect to specific M5 units. I know you can turn mesh off for devices and that helps a bit, but it doesn't seem to guarantee that the device will always connect to the same M5 unit after power cycling. 

Can TP-Link please allow users more control over their systems, as the automated way the M5 system works at present doesn't work as well in the real world as it probably appears to in the test environment.

I've picked up an S4 three pack.

I have the same issue. I have one unit at the back of the house as the main deco into the internet. Hallway deco is at front of the house and the two show a solid full link between them.

Directly above the Hallway Deco is my front bedroom where my Office deco lives. This unit is connecting all the way to the back of the house and the throughput speeds reflect it.

I get 240Mbps on the main deco (internet is 250Mbps), i get 190ish on the hallway and then 10 on the office,

The office one has my network switch connected to it with my wired devices. Typically would be this link that's running the slowest.

There is NO way the signal from the backroom is stronger than the one from directly below.

Is this a true mesh?

I'm about to open a support ticket.

I have this same issue.  One side of the house has the Internet connection with main deco, 2nd deco in the middle of the house, and 3rd deco is in the garage on other far side of the house.  Garage always wants to connect to the far away main deco and skip the 2nd middle one.  Its so frustrating. Please let us select the path!

I have found that the connections can vary from day to day. When restarting after one of our relatively frequent power outages, all the remotes always start out connecting to the main Deco. In a day or so, connections readjust to more of a appropriate mesh.

                      Main

                    /            \

Dining Room                 Upstairs

           |                                  |

Cabin Living room         Cabin bedroom

Several days later, I may see a new set of connections, also an appropriate mesh for my home/cabin, though slightly different.

                         Main

                      /             \

Dining Room                Upstairs

                                             |

                                     Cabin bedroom

                                             |

                                     Cabin Living room

It may not make any difference for anyone else, but I suggest checking your connections again after a day or two. You might also try defragging your hard drive.

Oops, defragging was the Customer Support universal solution of a different era. I meant, reboot all your Decos, starting with the main one and continuing in order of closeness.

@RandySea2 it makes sense that connections change. Especially with higher frequency ranges like 5ghz the connection strength is more sensitive to things like air humidity. So when for whatever reason the deco unit restarts or disconnects it might or might not find the power threshold to the main deco good enough to connect with. 
 

the main problem though is that there seems to be a strong preference for each deco to connect to the main deco instead to its closest neighbor. So in essence, deco acts as a star/hub&spoke design, not as a mesh. 

@Olafmol 

I agree that there is a real problem that TP-Link has ignored despite more than a year of reports.

If I had known this before I bought the Decos, I would definitely have bought something else. But since it seems to be working adequately, if unpredictably, for me, I'm pretty much stuck.

I certainly would not recommend the Deco to anyone else.