If you talk " С ума сойти!" this means,rapture.
If you talk "С ума сойти..." this means, mistrust or even dislike.
In general, few people know colloquial meaning of this expression ... This is just an expression of strong feelings and big emotion. In Russian colloquial vocabulary many such expressions)) Similar expressions are "Офигеть!",Обалдеть!,Жесть! and other...=)Maybe you've heard these expressions...
Tell me that I misspelled wrote,please.