They're actually quite cool. It's marketed as retrofitting keyless entry to your old car, but it can be used for any purpose - like controlling garage door openers.
It has decent range and two generic relays (matching the open and close buttons) that can be used for any purpose, not just 12V systems.
The only miss is that the sellers language skills are a bit limited, and it's hard to get a clear answer to how the device operates. Luckily for you, I've taken the chance and bought one. For your convenience here's the user manual / schematics: