Are they good to work with?

Can't say -- have never used one. Let us know how it goes.

Some time ago Silicon Chip carried an article on how to wire up a 12V downlighting transformer as an autotransformer to reduce mains voltage by 12V to increase the life of incandescent and fluorescent lights. The wiring arrangement was such that the full lighting current was carried by the former secondary of the transformer in the autotransformer arrangement. So the downlighting transformer, which might be rated at a few hundred watts AC, could carry the full lighting circuit current when wired as an autotransformer.

On this principle it should be possible to rewire a transformer with multiple secondaries with a range of voltages from say 10 to 40VAC, with suitable switching, as an autotransformer to provide multiple near-mains voltages. Would be suitable for providing the right voltages for items such as soldering irons, but also for valve radios.

In my case my mains has gone as high as 255V, but at the moment is under better control, going from 238V up to 248V, still significantly above 230V nominal mains voltage. I thought of building such an autotransformer arrangement when the mains was going over 255V (and tripping out my solar power inverter), and interfering with correct voltages in a radio I was working on (adjusting a high tension voltage divider).

I read somewhere (might be Silicone Chip) that the proliferation of solar panels which feed the grid is causing significant fluctuations in mains voltage. Sounds plausible.

"solar panels which feed the grid is causing significant fluctuations in mains voltage"

As you do I went online to find out why my inverter was tripping out. The usual tripping voltage is 255V; a solution to this was to open up the inverter and set this voltage to 260V. I wasn't inclined to interfere with the inverter, but others would. Perhaps someone in the local area had reset their inverter, or had just had an inverter installed with an incorrectly set trip voltage. Or perhaps a local pole transformer was faulty or had the wrong tap set. I will never know, but a couple of weeks later the voltage dropped to a slightly lower level than it had been, generally 245V.

This shows an autoformer voltage reducer hookup. Though this one is for 122V to 116V, the theory would work for reducing 240V.

I've never heard it referred to an 'inverse boost' transformer. Usually called a buck transformer.

Drawn another way it would have the two windings in series with 240V across the two windings and the tap taken off the junction of the two. This makes it look like a traditional auto transformer. (Had to do this mentally to make sense of it.)

Ok GTC will do.
Now that is an interesting arrangement. I have a few 240 to 12 volt and 6 volt transformers, I might experiment and see what I can get?

Guys, just remember earthing and that fact that 240V delivers a bigger wallop than 120V. That powerboard would not be a satisfactory arrangement in a 240V environment.