I set this up to automatically water my Christmas tree. Everything in the kit worked perfectly: I have no complaints about it at all. However, in this project, I discovered that these pumps do not prevent passive backflow (or forward flow) through them. Because of this, if you place the pump in the bottom of a jug of water next to your tree, and you run the tubing from the pump into the water in your Christmas tree stand, once you fill the tubing with water you have created a siphon system that will automatically equalize the water level between the water jug and your Christmas tree stand. If you turn the pump on and it lowers the water level in the jug below the water level in the Christmas tree stand, as soon as the pump turns off, the water will just passively flow back into the jug. At that point, you no longer need to power the pump: as long as you keep the jug of water full, the water will passively flow into the Christmas tree stand.

For clarity, I purchased the "WayinTop Upgrade Automatic Irrigation DIY Kit Self Watering System for Garden Plant" (since Amazon seems to lump reviews together for related products).

Overall, this is a nice little kit for the price. You'd be hard pressed to get all the components separately for this price. Their tutorial is on GitHub and they sent me a link. (If you know your way around GitHub it's pretty easy to find.)

A couple things I found. As someone else mentioned, their wiring diagram has a small issue. There should be a line from the battery to VIN, and the line from VIN to the relay board should actually come from 5V.

Their code is redundant and is not very clear without intimate knowledge of the hardware. Also, the default threshold value of 750 resulted in it never turning the pumps on because my sensors read 600 bone dry.
I submitted a PR (#2) with a cleaned up version of the code, so feel free to take a look at that and use it instead.

I had an issue with one of the sensors and contacted them through Amazon. They were unable to replace the sensor, but told me I could find one for $5 on Amazon, so they refunded the cost of replacing the one sensor, which I found to be a pretty decent solution. Especially since it was much faster than waiting for a replacement to be shipped from China!

I contacted the seller and they supplied me with the code and documentation within 1 days time. I did have a problem with the relay board and again they responded to my request for assistance within a few hours. Operator error. The kit includes everything you need for a basic automated watering system. The only thing I'll add is a water level float switch in the reservoir to tell me when the water supply level is too low to run the pumps. I purchased an automated LED grow light system earlier and this was the next area needing automation. I am happy with the quality of the parts in the kit and am very happy with the support. The kit is setup for 4 plants (4 relays, 4 sensors, 4 pumps). It can be expanded. The limit seems to be the analog pins available to read the moisture sensors. For the provided UNO it is 6 plants. If you have access to a NANO, it has 8 analog read pins so you could support up to 8 plants. My next task is determine the size of the water reservoir needed. This is completely dependent on the number of plants and their size and water needs. Each plant has different needs and the system will adjust to handle most plants.

Reasonable basic setup to run a few pumps with sensor data.
The circuitry could be improved. The USB disconnects when the motors are turned off. This could be fixed with better isolation in the relay board and probably a few diodes.

I am using a nano and a nano breakout to simplify wiring of the sensors.
The sensor wires are short so I am using "servo extensions" to get additional distance between the plants and the arduino.

This is a kit that comes with everything you need except a battery compartment or power supply which you'll need to power the water pumps. I was able to complete the entire kit in a couple of hours. I was able to get the instructions and the code from their Github link. The only issue I had was that finding the instructions in their Github repo took a little digging but once I found them I was on my way. One thing to be aware of is that the pumps are submersible so you don't have to attach any tubing to the intake side of each pump. For the price this kits is a great value. They even give you an extra pump and tons of extra wires. Also, the code is very very simple. I did have to adjust the dryness/wetness level in the code but that was very easy after reading online documentation for the sensor. It was very satisfying putting together this very useful kit and there are a lot of possibilities from what you will learn.