There have been studies where AEDs were put in airports and such and lay persons successfully followed the directions and saved lives with them. They're very dummy proof at this point: place sticky pads where a picture shows you, turn it on, and follow the voice prompts. Even if you don't know CPR, you can help by trying this.
As further safety, the DSP software has a bunch of safeguards to only shock the types of bad rhythms it knows how to correct, so it can't hurt a normal person if the AED is mistakenly applied; it can only help a v-fib one.
Training won't hurt; the Red Cross course for combined CPR and AED is only a few hours. But there is benefit from none at all.
I've personally brought someone back with an AED and the experience changed my life.
https://www.resus.org.uk/pages/AEDtrnst.htm
They are still few and far between, outside of big railway stations I haven't seen any in public. I think one of the issues is people are unfamiliar with how easy they are to use.
All workplaces (above a certain, fairly low, level) are required to have first aid trained staff around at all times. The training only includes CPR though; adding AED training seems like an easy way to make people a lot more aware of them.
Some here are in a cabinet that phones emergency services when the AED is removed; that's an especially good feature.