I decided to watch The Terminator (1984) again before watching Terminator Genisys. This turns out be more worthwhile than I initially thought, as it enables me to fully appreciate the remake of the original scenes in the new movie. This new movie reboots the franchise and changes what happened in 1984 by creating a different timeline. Basically, when Kyle Reese travels back in time to 1984 to protect Sarah Connor, the mother of the future leader that will lead the resistance against Skynet, she already knows everything, and had a Terminator that protects her against other terminators.
This change is fairly interesting since it creates a different setup. However, as the movie progresses the story becomes generic. While it is still a competent action movie, other than the new bad guy there is nothing new to see here. On this basis I like Terminator Salvation (2009) more, but unfortunately it is erased from both the original and the new timeline.
Note: There is an important extra scene during the credits.
P.S. The 1984 original explained that the time machine was destroyed after sending the terminator and Kyle. In this movie it is implied that it is not destroyed, and it also shows that the time machine can send more than one person at the same time. Why not send a team, or the whole army?!