I use it.
Saves a lot of time to order by part number over the phone than waiting for them to look it up by CPL# or sn of truck. They then can tell you if it's in stock or will call you when everything is in.
Also will show if a part number is superseded with new a part number.
For example my Aug 02 built 03 - the head gasket is #3958645 now has a newer improved gasket #3954876. The parts guy will normally give you the orginal gasket unless you know and ask for the newer.
For $50, to me it was worth it to have the info available.
Last year when I put head studs in and built a set of twins one phone call and all gaskets, injector shims, orings, oil return fitting, and all other things cummins could supply that I needed was ordered by part number. Took 7 working days and I had everything I needed before I started.
Bill