You must really like looking at high plastic seat backs if you liked GWR's horrid Mk3 interior because that's most of what you could see.
Do you realise some non enthusiasts say it "luner"?
A subject done to death before but Only the High Density sets were bad, and that was done to meet passenger demand and growth. The other option FGW had was keep their HSTs low density and ignore the complaints from the Bristol and Cardiff commuters about getting a seat. The low density sets (and particularly the 425XX’s) were perfectly fine to travel in - and I enjoyed them too. The same Grammer seats seem fine in ScotRails “Inverness” 158s to little complaint.