I'm only nesting a few levels deep. And I know it's like nesting views. And when I execute the function via:

SELECT TOP 1 * FROM dbo.udf_...

or

SELECT COUNT(*) FROM dbo.udf_...

The performance is reasonable. It's when I do the

INSERT INTO <table> SELECT <column-list> FROM dbo.udf_...

that the performance goes to hell well beyond what seems reasonable in comparison to if the same ITVF had been written to utilize # temporary tables that are pulled together using similar logic as what occurs using the nested ITVFs

Respectfully,

Darryll