Problem with a scalar function

I am attempting to provide a custom function that should replace provide an IIF function (Access SQL) in an upsized database.

I have defined a new scalar function as follows:

ALTER FUNCTION [dbo].[IIF]
(
      -- Add the parameters for the function here
      @check bit,
      @TrueVal sql_variant,
      @FalseVal sql_variant
)
RETURNS sql_variant
AS
BEGIN
      -- Declare the return variable here
      DECLARE @Result sql_variant

      -- Add the T-SQL statements to compute the return value here
      SET @Result = CASE WHEN @check<>0 THEN @TrueVal ELSE @FalseVal END
      -- Return the result of the function
      RETURN @Result

END

I want the IIF function to take 3 parameters, a boolean value, a value for the true result and a value for the false result.  (the results can be strings, numbers... so I have declared them as variants and require the function to return a variant).

The function compiles (Verify SQL Syntax) without problem.

In a view I have the following:
SELECT      dbo.IIF(dbo.Fibu.[Beleg ID]<600, "Lower", "Upper") AS Expr1 FROM.....

If I then use the 'Verify SQL Syntax' option I get an error - Incorrect syntax near '<'


Just for experimentation I then modified that to
SELECT      dbo.IIF(dbo.Fibu.[Beleg ID], "Lower", "Upper") AS Expr1 FROM.....
and now get the following errors:- Invalid Column Name 'Lower', Invalid Column Name 'Upper'


Obviously I have a serious misunderstanding.
1) Can I add a scalar function to the database and then call it inside a view?
2) Assuming the answer to 1) is yes then what is wrong with my attempt above.