I completely disagree.
Oh no, not at all.
Disagree to some extent
I think I would probably have to say no.
I can see your point, but.....
Neither agree nor disagree
Well, I think there are valid points for both.
Actually, I think it depends on the situation/country/person etc.
It seems to me that there are two sides to consider.
Agree to some extent
To a certain extent, yes.
I totally agree with that.
Yes, I think that's absolutely right.
The Official Cambridge Guide to IELTS for Academic & General Training p.385