Never used a proving unit ever until recently
I tend to carry a Martindale AVI (after my Steinel broke) also have a clamp meter , multimeter and a Drummond all in the same bag, but only use the AVI's for checking voltage is present etc
One thing that annoys me is most makes the probes are quite chunky and some isolators don't have holes big enough to make contact
Previously always used known supply, seemed like a large outlay for something rarely needed for the type of work I used to do as there was always a socket or something that could be utilised
However moved into more industrial work a few years ago and had several occasions recently on sites where using a known supply wasn't allowed as it usually meant opening a panel etc and a proving unit was needed ,
Daft rules I had to prove dead then lock off etc only to be turning panel back on after dead testing to faultfind components which was allowed !
Spent 30 mins looking around at various sites and saw a used RS branded proving unit on eBay
I bid a fiver and won it, so with postage came to just under 8 quid , new battery fitted worked fine and probably the best bargain I've ever had tool wise!