I’ve got an old 27.2mm Alien suspension seatpost… the clamp is fiddly to use but never, ever, shifts. I’m sure it’s picked up some play (or stiction?) which makes it jerk occasionally when riding, but I’ve never been able to reproduce it in the garage to check.
…
I was typing out to offer it to you for something nominal as I’ve not used it for a while now, but – sorry! – I just went down to the garage to check on it and see what bits I actually had, and discovered it came with servicing instructions (I’ve still got the box from 20-odd years ago!), so actually you’ve inspired me to take it apart and try and sort it out instead to use on my current road bike!
As I appear to still be quite attached to it despite the small issue, I’d have to say it’s probably worth getting one 😉