Terminator Genisys feels like the franchises own 'Days of Future past', in which the slate is wiped clean. It's also kind of like the TV series, the Sarah Conner Chronicles in a way as well, with its multiple timelines.
So Terminator Genisys involves a semi-reboot of Terminator 1, in the sense that the events that happened in Terminator 1 sort of happen but also don't because someone else was sent back into the past to change things.
Like T1, Kyle Reese is sent into the past to protect Sarah Conner but this time, things go FUBAR. Due to certain anomalies, Sarah Conner has another Terminator that was sent back in time to protect her when she was 9, ala Terminator2. Add that to the fact that John Conner is now a terminator-human hybrid and basically...the timeline is a mess. However, the mission remains the same; Stop Skynet at all costs.
Which is kind of one of the things that's really holding the Terminator series back I feel. Terminator 1 was great. T2 was good. T3 onwards however...eh, the less said about T3 the better. The issue with the Terminator franchise is that it hasn't changed or progressed since the original 80s movie. The first T1 movie was basically about thwarting Skynet by making sure John Conner was born. The 2nd was about thwarting Skynet by making sure John Conner surviving and throw in the added message of there being no fate but that which we make our own. Then T3 was about beating Skynet. T4 was basically Skynet wasn't thwarted. T5 is AGAIN about beating Skynet but then to do so, we'll loop back to the T1 timeline to do so, but with the added bonus of John becoming a terminator as well.
So at this point, I feel that the Terminator Series is stuck in a time-loop of its own making. What can they show us really that doesn't involve Skynet or basically rehashed versions of T1 and T2? Because somehow it feels inevitable that Skynet will rise again no matter how many times they try to thwart it or destroy it. Sure, T5 ended peacefully enough with Skynet having seemingly been 'destroyed' but if there was a T6, you KNOW that Skynet would somehow be back.
With that being said, I'll say the film itself was decent but it really really doesn't take you to any new places that you haven't seen in Terminator films before. I did like the future scenes; where they showed the future Rebellion fighting the Terminator armies but there were too few of those. John Conner was a pretty interesting character and probably the MOST likeable version of him that we've seen so far, having been portrayed with a sense of wry humor. It suffers from the same problem a lot of time-travelling stories do now that they've officially gone into the whole characters from multiple timelines that co-exist in the same timeline schtick.
There was one part that bugged me though, the fact we never got to see who sent the T-800 back to protect Sarah Conner. They never reveal it in the film and that's just annoying. Especially since it doesn't happen anymore in the 'new' timeline and we'll probably never know. Unless it was revealed in some comic tie-in.
Also, personally I felt the ending could have been much better if instead of the whole John Conner is now evil schtick, they could have had made it so that instead of being fully subservient to Skynets domination, John Conner would secretly have been helping to make sure that Skynet died. If he was the one who sent the T-800 back to help Sarah Conner but wiped its memories so Skynet wouldn't know and only revealed it at the end that everything was a Xanatos Gambit to finally defeat Skynet. That's just my personal opinion.