By the time I was born I think most if not all the mark 1s had left mainline duties, so my only experience of them is on heritage railways.
They have amazingly comfortable seats compared to modern stock, although it seems like all new trains are less comfortable than their predecessors. I do think they ride quite poorly though, no where near as smooth as a mark 3.
And that's the ride at preserved line speeds (no quicker than about 40mph). I've no clue what it's like at higher speeds. The fastest I will have done on them is about 70 when I was on a rail tour hauled by Leander, however I was only 8 and so remember very little of it.
I'd take the mark 3s and 4s over them, as unlike many others who may be reading this, it's those that I have nostalgia for, whereas I have no attachment at all to the mark 1s (or the mark 2s, which I have even less experience of).