I'm on an IET now going up the B&H - the ride quality is far and away better than when the HSTs ran along here. It's firmer and a little bit more juddery, but certainly not the roller-coaster wallowing and teeth-banging lurches we used to get with the Mark 3s.
They were better when they weren't so worn out, but they did need treating like you were in a small boat if you wanted to move around even when they were new... ( fortunately I've been sailing for most of my life! ). The alternate 159s are just lovely, I don't get how we can't duplicate their success. Admittedly they don't do 125mph.