I can't comment myself as I fitted a proper water softener although I am now contemplating fitting a magnet jobbie to the main toilet of the house which I didn't plumb into the soft water side as it's used so regularly but now has a little bit of hard stuff building up either side of the bend...
It wouldn't be hard to run a controlled trial:
Get someone to install two taps. One has a magnetic device on the pipe run, the other doesn't. The pipes are boxed in so you can't tell which.
Each tap fills a separate kettle. Any time you want a cup of tea, you fill both kettles equally from the respective tap. You boil both kettles. You make your tea from one or other kettle. You throw away the water from both every time.
The level of scaling in the kettle should be a reasonable indicator of if it makes a difference...
Theo