I understand this is a subjective question, so I apologize if it needs to be closed, but I feel like it comes up often enough for me to wonder if there is a general preferen
I favour the if (!value) style at least for evaluating variables or common properties like Page.IsPostback and the like. For anything more complex I tend to parenthesise the expression like thus:
if (!(SomeType.SomeProperty.CallingAMethod(input).GetSomething.BooleanProperty))
Just to draw a little more attention to it.
All in all, it's an argument for Perl-style unless and until keywords.