I've done Paddington to Reading many many times in an IET.
I don't see what the issue is, the ride is not as good as a HST but it's pretty good. Much better than a Voyager or Meridian and certainly a completely different ballgame to the 345s which are by far the worst riding trains I have been on
As per other posters on this thread - it is extremely variable - and from my own experience I disagree that "fearing for your life" is hyperbolic to describe what IETs occasionally seem to do.
I've traveled this line probably a thousand times over a few decades - plus the length most other UK mainlines albeit much less frequently, travelling on business. The only time I've momentarily feared for my life on a train was on an IET between Reading and Paddington.
It was always fairly bouncy and lurchy on HSTs to be fair. Also, in general on IETs it varies from surprisingly smooth to broadly rather harsh and jittery with a tendency to hunt and occasional (presumed) bottoming out of the suspension.
However, from time to time and seemingly at random, things can go to a different level. A few years ago, I was on an IET on this stretch which started prolonged violent lateral and vertical shaking plus alarmingly loud banging from bogies/underfloor equipment to a point where I'd already located the emergency handle. I was hovering around the trigger point for pulling it despite a very British fear of public humiliation for overreacting. What happened at Eschede came to mind. Other passengers were popping up from their seats wide-eyed and ashen faced and a few moved to other carriages.
More recently, most journeys have been relatively smooth. Also, I've increasingly tended to sit in the front carriage of the train for other reasons, with the driver not far away. My logic is if the person with the brakes literally to hand is conscious and not bricking it, it's probably fine.