On Amazon 30 odd users give fuel fit a 4 - 5 star rating, reporting it does what it claims. The cost is modest, I think a little £5 bottle will treat 40 litres, which is a lot of sawing- more than I do in a year.
Here's their video explaining what it supposedly does, if it saves binning a tank full of fuel (again, Ratty)- its worth the extra few pennies. I suspect many people (including myself until told by Radmore & Tucker) are unaware that petrol can rapidly degrade and can 'go off' in as little as 30 days unless treated. Its not an issue with a car, which most people top up with fresh fuel every week or two, but lawnmowers, chainsaws, etc are used seasonally and often left unused for sufficient time for residual fuel to lose much of its 'uumph' and cause starting problems.
http://www.briggsandstratton.com/eu/en/support/videos/other-engines/briggs-and-stratton-fuel-fit