Is there any difference between this and the SQL function ISNULL? For simplicity and to avoid 'mindclutter' why would we not call it the same? I'm just thinkin'...
SSRS deals with NULL via the IsNothing this is VB-sentric as SSRS is VB-based rather than SQL based when it comes to the underlying code.
So, saying IIF(IsNothing(Fields.MyField.Value),"1","0") would be the same as:
IF (Fields.MyField.Value IS NULL)
SET @X = "1"
SET @X = "2"