I had a day trip from Manchester to Stoke and back to sample them shortly after they were introduced. I remember being impressed by their ride quality, sheer length (seen to good effect on the curve at Cheadle Hulme) and their rather vivid seat upholstery - bright blue in 2nd, orange in 1st! The big controversy was the non-alignment of windows and seats in 2nd and the smelly brakes had yet to be fixed.
I did the same thing when they were brand new (although being an impecunious student only managed Piccadilly to Macclesfield and back) and had exactly the same first impressions - even down to the "Wow, these coaches really are long" moment passing Cheadle Hulme.
The only other thing I noted was the automatically-opening internal doors between the saloon & vestibule, activated by some sort of foot treadle under the carpet IIRC. "These won't keep working for long", I thought at the time, and I think I was correct.
Modern Railways soon had reports of frequent failures of the opening mechanism and luggage-carrying passengers having to struggle with malfunctioning doors manually.
When more Mk.3s had been deployed onto the WCML - maybe in 1976 or 77 - one set ended up working a MAN-EUS/EUS-GLC/GLC-Manchester Victoria diagram. Since the Mk.3 stock was based at Longsight, and this was well before the Windsor Link, the evening 1M40 arrival into Victoria was dragged back to the Ordsall Lane area (possibly by the Victoria pilot loco), then reversed and proceeded via Castlefield Jn. and Oxford Road home to Longsight. Possibly there was another roster covering the morning Man. Vic to Glasgow direct (non-portion) train which then formed a GLC-EUS service?
I mention this because the train of modern, shiny, lengthy new Mk.3 carriages (BG & buffet excepted) always looked very out of place passing the forest of semaphore signals around Bolton East, or amongst the 2-car Cravens and BRCW DMUs belching diesel fumes at shabby old Victoria.