I’ve done plenty - chemistry PhDs in the UK primarily with a few in Europe. Our system is a closed viva with one internal and one external and is meant to be a good test of the candidate - it’s considered a poor show by all involved if they’re not put through their paces, although in truth it would take something really untoward to fail at that point [it would need a combination of a very weak thesis plus a bad, CBA, attitude in the viva, I’ve never encountered this myself].
If I’m external I’ll read the thesis properly, as I will be leading the viva - internal I’ll start with the candidate’s papers and just speed read the thesis as that role is more supportive and less interrogative of the candidate.
I like the viva-as-celebration approach. Give the candidate a few digs by all means but basically give them a chance to shine and show off their research. For this reason I dislike externals who try and trip up candidates to try and show who the smartest person in the room is, and likewise extended questioning of strong candidates on background theory I find to be aggravatingly pointless.
The worst experience I had was when I was quite new, and a colleague asked me to be internal for one of her students who (she felt) had let her down. He’d worked his way through a long synthesis but had failed to complete the molecule in the last steps, and was a bit dissembling as to the real reasons behind this. He was a very solid scientist, though, and the PhD was basically fine. Out of spite, she brought in the meanest most pedantic old-school external she could find. Student went to pieces under the sustained bollocking, and the external wanted to fail him. There was no way this was right so I had to argue to pass him by fudging a regulation that meant me re-examining the student on my own [the thesis was fine but had not been defended properly], which we agreed to and I subsequently did. An unpleasant experience, although a useful insight into the character of my colleague at the time.
Viva systems and protocols vary widely - I’m doing one in Sweden in the New Year where I have to give a presentation on the candidate’s work to an audience. This seems pretty funny, given the attention some externals typically give to a thesis, so should be an interesting process.