Azimux has a good point, some people just won't use it. However it's perfectly acceptable to me. Read it as: "I should say (it is right for me that I say)... 'not'." It's a blunt disagreement, but in a formal structure. It's difficult to say when you should use it, apart from a formal setting. You really need a good sense of English to know when to use it. You can easily use the common "I don't agree", but don't dump "I should say not" from your phrases. In some circumstances it's much more useful than the basic forms.