TBH all of the 15x, are getting very long in the tooth. Sat on 153 and 156 today and they just seem ridiculously old fashioned - and I can remember the excitement as a child when they first started, so modern!
The noise levels are appalling. The 75mph top speed is too low for today's railway, as most routes involve at least some running on lines with a higher line speed so capacity is being wasted. There isn't enough legroom. I usually find myself inhaling exhaust fumes, which is totally unacceptable.
They're just dreadful. They were cheap and nasty when introduced, but were arguably appropriate for a system in managed decline.
I think they are completely unsuitable for today's railway. Really the whole lot ought to be scrapped and replaced with modern stock from CAF, Stadler or Bombardier. The industry would do well to work collaboratively and specify 2 or 3 designs to replace them. 1 local commuter type, 1 longer journey regional type. All bimodes for future electrification and to make use of current electrification where it exists.
158s are just about OK, although the EMR examples are a bit scruffy (the winged headrests are particularly uninviting covered in grey crud). They might as well be replaced too as they can't carry on forever.