I've had mixed success with this. I bought one of the plugs and successfully soldered on the headers, flashed it, and have it working.
Bought a second, flashed it, but could never get it visible on my network with the flashed firmware. Then bought an 'inline' sonoff switch and can't see that in my flashing software (which suggests my soldering might not have been up to scratch). My furniture restorer wife has now burnt out my good soldering iron using it for something inappropriate so I'm a bit stuck.
Is there anyone out there with good soldering skills who would get my couple of Sonoffs working in exchange for a few pints?