The 420EX was originally introduced by Canon as a cheaper alternative to Speedlite 550EX while it could also perform as a companion, by being able to accept remote commands from 550EX in Slave mode.
Having had some frustration with early YongNuos, I had purchased 420EX used, at a very attractive price, exactly for its slave function.
Beside that, it worked very well on top of EOS 400D /Digital Rebel XTi mostly as a bounce flash. However, there are some incompatibilities with crop factor digital cameras concerning the zoom ratio translation as the flash unit was designed with Canon film cameras in mind.
