Very simple and cost efficient way to connect lights for a smart home. I use these in the ceiling to voice control my lights with google home. I have it set to automatically turn on when the light switch or power is turned off/on, just in case my internet goes out (I do still need lights). Love this feature.... You can do this in the app once you get it set up.

I had to use the manual connect mode since the traditional method didn't work, but it was very simple and relatively quick to connect. (About 1 minute to find connection and fully setup, installation was the longest, about 15 minutes each.)

I would recommend these to anyone. It is easy to connect the wires even if you're not too familiar with electric wiring. I do have to say, it can be tricky to fit them in the ceiling. I thought with the shape that it would fit inside the lighting can in the ceiling, but it is a VERY tight fit. I found it is easier to place above the lighting base in the ceiling behind it. (Harder to get to in the future, but hopefully I won't need to readjust it again.)

I was a little late to this game, not having discovered the Sonoff products until probably 2018 or so, but I've made up for lost time: I use these all over the place to "add smartness" wherever it's needed: desk lamps, light switches, garden watering, holiday shows, the dust collection system in my wood shop, and controlling the garage doors. I always keep a few in inventory because you never know where one will be needed next. The only ones that have ever failed were due to human error.

1. not for regular joe, great for joe that has soldering iron and no problem tearing into a 4 gang box with 2 dimmers and 2 3 ways.
2. 3 way switches, particilarly with a dimmer, are really tricky. you need at least two of them, and a external realy also if you want to alter between off/dim/bright remotely. trust me,
3. please check the connections on the board, I've had several (a pack of 6 faulty was assembled out of perhaps a hundred or so ordered) .. while fixable, just send it back, not worth the risk,
4. it does NOT fit into a single gang outlet unless you mean at an angle, sideways, but with good planning and cleaning up the wiring, completely posssible.
5. You'll break open every one of them to get at the GPIOs if you want the original switch to do anything. Reuse them, leave the ground (bare) connected, connect the internal ground and (best choice) GPIO 14 (sensor).
6. Use SonOTA or equivalent, don't mess with serial programming unless you have a compelling reason.
7. Use grouptopic, switchmode, powerretain, and all the other commands to get it to act exactly like the old one did before going on and getting fancy.
8,. make sure you can still get at the button with a pencil (or other insulated thing) by just removing the cover and poking in there. You'll probably need to at some point.
9. Do everyone a favor and get a decent wifi AP for all of this, and then an OrangePi/RaspberryPi/whatever to run your broker and whatnot directly via that AP.
10. getting MQTT to work is easy. getting anything else (HomeBridge/Domoticz/whatever) is a nightmare. But do hard part first and clean up your mess.

TL;DR get some Alexa gizmo if you just think it's cute to dim the table lamp via iPhone. Get these if you're the kind of nerd that thinks blinking the whole house is a fun idea. Yes, includng the bathrom.

The hardware for these Sonoff switches is great! I load the Tasmota firmware on them using Tasmotizer and connect them to Home Assistant using and MQTT server. I use these switches to control many things around the house like sound machines, baby monitor, patio lights and also Christmas lights. It is very convenient for Christmas lights because you can have it turn lights on and off based on sunset/sunrise. To make it easy to use these switches to control various items and appliances, I purchase 6 foot extension cords at Lowes/Home Depot that are a couple of bucks each and cut them in half and wire this module in the middle of it.

I bought these switches to help automate our living room lighting, specifically for a number of plug-in lamps. In order to install these, you need to cut the cord and expose the ends of the wires. You also need to determine which of the lines is hot and which is neutral. Since most plugs are "keyed," with the larger prong being neutral, you can do this relatively easily. (I still used a meter to double-check.)
The wire connectors are very secure and a clamp on each side provides strain relief so they wont pull out.
These switches have a manual button on them so you can turn the device on or off manually as well as WiFi-controlled.
The Sonoff brand uses the Ewe-Link app to pair with and control the switches. I'm also using an integration to control them in Home Assistant. The pairing is easy and the app (and Home Assistant) work very well with them. The switches also have an LED that lights blue when the switch has power and red when it's on.
There are two caveats: First, the switches make an audible "click" when switching on or off. (It's the relay. There's nothing you can do about it.) Second, if the power goes out, the switches start back up in the "on" mode. So, if your lose power, all your switches will turn on when it's restored.