Amusing timing as this comes from traveling on one for the first time in a few weeks. They seem susceptible to pick up a resonance where they wobble from side to side quite rapidly on occasions, you don't feel it much but you certainly hear it, I wonder if that's one of the things people complain about. Once on a smoother bit of track, though, it disappears entirely. Fundamentally though I still think they are a lot more pleasant to travel in than MK3 EMUs even as a standing passenger which I am now. Back in the days c2c ran the Laindon terminator services more regularly I used to feel quite comfortable during my 25 minute seated journey back to Upminster. Being in the middle of a 3+2 is no more fun than it usually is but otherwise I think people complaining about the state of the units are expecting rather a lot, especially as they're often the same people complaining about the specification inadequacies of newly introduced stock, e.g. 387s, 700s and so on.
