In short, worthy sequel!
I, personally, enjoyed the movie and despite the critics the movie is a good enough sequel to the previous ones.
DF had funny scenes, new characters, change of place and a next to impossible to beat Terminator!
And the main reason, AFAIK, why people diss on TDF is the first 5-10 minutes (spoiler).
It is worth watching, and I did it mainly for Arnold and the return of Linda's Sarah (and since I like franchise movies).
Don't let the critics give you a hard time deciding should you watch it or not, if you want to watch it then go for it!
The end of the movie was emotional and sad, which makes it even better that the other ones (besides T2), and more on par with T2.
I get that the last Terminators between 2 and DF were flops but they tried to extend the franchise and to do something different.
PS: I am very open-minded about things so, this comes straight from the passion for movies I've got!
So, in conclusion, I liked it and it's a well-made sequel,
and remember,
*The future has not been written. There is no fate but what we make for ourselves.* -Terminator franchise.