Because I often get bored at work... one day I decided to learn all I could about WD-40. (Seriously. I get THAT bored at work.)
There's a lot of misconception in this thread -- "it's only a water displacer"... "it can't lubricate".... "Astroglide is better than the cheap drugstore brand stuff". The truth is actually kind of interesting.
WD-40 was developed as an ancillary part of the Atlas missile program. The Atlas program needed a compound that could be used to easily and thoroughly coat parts of the rocket that needed to remain absolutely water-free. But it needed to meet certain tolerances and requirements because of the ultra-high-tech application involved. (For example, a lot of oils will actually explode if brought into contact with liquid oxygen. That can ruin your fueling process real quick, I tell you whut...)
Here's rub #2: if you want to coat something thoroughly and easily, you typically want something with very low viscosity that can be sprayed, and which can flow easily over the surface. However, if you want to protect something long-term, that's exactly the opposite of what you want -- you want something really viscous that will stay put wherever you put it.
The reason WD-40 was accepted, and has been so successful, is that the formula does both. It's all hydrocarbons, but basically part of it is very non-viscous and highly volatile carrier with a more viscous and non-volatile protecting substance built in. In the can, everything is mixed up, and the substance as a whole is pretty fluid and easy to spray in coats. When it gets out into the air, though, the volatile carrier evaporates away, leaving the viscous water-repelling oil to coat whatever you just sprayed.
So addressing those misconceptions:
-- Yes, WD-40 is first and foremost a water displacer. But it's a water displacing oil -- and like all oils, it has some lubricating ability. It just doesn't have the same lubricating ability as special-purpose lubricants like silicone grease and what-not, and the lubricating ability has a pretty short half-life (the more carrier evaporates away, the more viscous the remaining WD-40 becomes).
-- WD-40 is great for cleaning metals that don't need special lubrication. It will clean virtually anything, because that volatile carrier is a good hydrocarbon cleaner (after all, it's designed to dissolve tough oils... like, say, the water displacing oil in WD-40) -- but it's going to leave behind a fairly viscous oil-based water repellant in its wake. And that can gunk up things that need to be regularly lubricated.
-- WD-40 does have the ability to act as a penetrating oil (i.e. something you'd use to loosen rusty bolts), because it initially has such a low viscosity. That low viscosity lets it get into cracks and threads better than more viscous materials. But it's nowhere near as good at that as a real penetrating oil, because a real penetrating oil doesn't evaporate away.
The bottom line is that WD-40 is a jack of all trades, but a master of none. It's good for emergencies, small jobs, and some routine maintenance tasks. It's not good for regular lubrication tasks -- for that, get a proper lubricant oil or silicone compound. Everyone should have some on the workbench.... but you gotta know when you should and when you shouldn't be using it.