SQL Function to transform rows into delimited text

CREATE FUNCTION CitiesAsDelimitedString
(
@Delimiter AS VARCHAR(5)
)
RETURNS VARCHAR(8000)

AS

BEGIN

DECLARE @str AS VARCHAR(8000)
SET @str = ''

SELECT @str=@str+CASE @str WHEN '' THEN CityName ELSE @Delimiter + CityName END
FROM Cities

END
---------------------------------------------------------

Usage:
select dbo.CitiesAsDelimitedString(', ')