The Boeing Museum of Flight once offered rides in a Merlin-powered P-51. I was first in line that day, and was smiling for a week afterwards.
From the book:
"One day I happened to be chatting near my home with an RAF Wing Commander (retired) now dressed in dark grey slacks and Norfolk jacket. Suddenly out of the blue and on its way to Coningsby was a fighter of the Battle of Britain Flight right over our heads.
We both stopped talking and looked up--the crisp steady
note of the old Merlin as joyous a sound as ever it was all those years ago. The ex-Wing Commander said absolutely nothing-- then he sighed. It was such a poignant sigh. I felt it must have invoked so many memories of the days when the Wing Commander was young, daring and vigorous and like a Knight of the Crusade had leapt onto the Spitfire and its Merlin to ride into the pages of history.
The silence was still uninterrupted. Thoughts and memories began to flicker through my mind. I remembered crashing badly and the terrifying sound of things being torn apart; closing my eyes as the ruptured earth flung pieces of metal over my crouching head; the peculiar smell of oil, petrol and glycol and damp earth hanging in my nostrils. Then the panic as I struggled out of parachute, harness and shattered cockpit--and then the silence. An almost deathly silence. As I prayed a word of thanks over the crumpled wreckage and the large black mass buried in the soft ground--once a powerful, gleaming engine--I heard this sigh. It may have been a pressure-relief valve, or glycol, or oil on hot metal, but in the emotion of the moment it became a sound I shall always remember. What better epitaph to a wonderful machine and a magnificent engine than to call my book Sigh for a Merlin?"
I would guess that, being hand-built, it would be trickier to keep them well-maintained, and require a particularly well-trained and skilled corps of artificers. The article does not get into it, but I wonder if there were differences in reliability between the Packard- and Rolls-Royce- built Merlins?
Mind you - in both cases recordings really don't do them justice.