This article is still in progress.
Simply put, as long as you can manage to put this adapter into your phone, then it is super convenient to change your SIM cards without physical swapping and rebooting. Works for both voice and data (2G/3G/4G/LTE). No it cannot dual standby, that's what a dual-sim phone does and you do can use this adapter in a dual-sim phone.
I bought the adapter from Newegg for less than $6, other similar adapters should work as well. I have tested it in my LG F6 and BLU NEO 4.5, work like a charm. To use it in my LG G2, which has a SIM door like iPhone, I have to leave the adapter outside and hold it by using a phone case (I usually don't use cases).