I have a sliding gooseneck on my HR28. That was part-and-parcel with the old-school roller booms of the 60's. Wouldn't replace it like that today, but since it's existing gear, we use it.
We don't use the roller function, but the sliding gooseneck with downhaul is a good thing -- to allow our Dutchman to be tensioned at rest, yet relaxed when the sail is hoisted. We release the vang and downhaul, hoist fully, and then tension the downhaul to adjust luff tension, just like a cunningham.
FWIW, We do run McLube on the gooseneck track, as it isn't as low-friction as it could be.
Paul Van Voorhees
Certified Tohatsu TLDI Technician
Mgr, Obersheimer Sails
Buffalo, NY USA