There are different ways of reacting to other people's opinions, depending on how well you know the person and whether you agree or disagree.

If you want to agree with someone you can say:

-That's exactly what I think.
-That's just what I was thinking.
-In my opinion, office workers are not paid well enough.
-That's a good point.
-I agree entirely.

If you want to disagree with someone:
-Quite right, I couldn't agree more.
-Yes, I'm all in favour of that.
-Well, my opinion for what it's worth...
-I can see what you mean, but ...
-I don't think so, because...
-I don't think it's a good idea...
-Maybe, but don't you think...?
-That's true, but on the other hand...