I loved this film. A lot of people hated it because the real Arnold wasn't in it. So what? there's more depth to the Terminator franchise than a 3rd story about Arnold protecting John Connor from a more powerful Terminator than himself.
Besides, Arnold is older now. He looks older. It wouldn't make sense to show a prototype version of Arnold's Terminator, that looks older than it did in the first Terminator movie... if that makes sense haha.
I thought it was excellent how they did it. A real nostalgic moment. I hope they re-do the effect again in the 5th Terminator film (if that's still going ahead

)
If you enjoyed Terminator 1 or 2 (more so 1, in my opinion, because the 4th film is quite dark like T1)and you haven't seen this film yet, then you're missing out!