Positive experience on pretty much all the 158s I've been on. It would be a tragedy if they did not outlive the other sprinter units by at least a decade, they are leaps and bounds ahead of the other units. I first remember using one from Sleaford to Lincoln some time in the late 90s, having been forewarned about the mediocrity of the rail service on the route so was pleasantly surprised by the quality of the unit. Of course, the 153s that operated the route almost every time subsequently were considerably less enjoyable :P
Used the SWT 158/159s down to Hampshire a few times and always enjoy the journey, sit back, relax, doze off for an hour. Also fairly impressed with the ATW 158s on the one occasion I used one although the return journey was very draughty and quite cold as a result. I haven't used an NR, FGW or SR version so can't comment on those.
In general there are a large number of multiple units out there that I think should really see major refurbishment to justify their use well into the 2020s, but the 158s aren't one of them. They're probably the oldest units I can think of where I'd be happy for them to carry on for the foreseeable exactly as they are, at least on the TOCs I've used.
That mechanical conversion 158 is very interesting - I'm surprised to hear the distinctive whine of a 158 is still present, I had always assumed it was the gearbox that made that noise.