It is like Marmite - you love it or hate it and there seems to be very little grey area in between!
I personally find it very useful - at various times in my life.
In My Youth
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On Scalextric cars, as it will dissolve old sticky oils. Its lubricating properties are 'ok' for a resonably competetive race, but PTFE is the way to get a competetive edge.
Bycycle repairs - chrome polishing and protecting - quick and practical. Excellent for washing ball bearings!
Young Adult
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Any owner of a mini (a real one not a new BMW type) carried the stuff - it was essential on a damp day and in driving rain. Even a rubber boot on the distributor and a plate stuck on the front grill did not protect the distributor fully. Water displacement is what it does 'in spades'.
When I purchased an unusable motorcycle from a friend, it worked to tame the front brake. The MZ front disc brake was so violent it had thrown the owner off the bike twice. I used WD40 to remove the dirt and oxide on the surface of the disc, reduced the efficiency of the brake and made it rideable.
Adult
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Now with a Son, I've gone full circle. Repairing Scalextric cars again (although I never REALLY stopped), repairing and cleaning trains, repairing his bike.
It is usually the quickest for removing glues and sticky c*ap from surfaces. It dissolves certain paints (sometimes works on toy cars), tars, tobacco stained radio grills. It needs washing off afterwards, but dissolves lots of substances quickly.
As for working on cars in general - it is usually there with me, like an adjustable spanner, big screwdriver and a pair of pliers.
But, like most things though, if used for the wrong purpose - or used too liberally, it causes problems. Keep it away from commutators. Keep it away from plastacine and blu-tac!
WD40 smells great
SEAN