WD-40 comes up in almost every lubrication thread on any gun forum in existence. No it's not a good lubricant.
I have found though that if people use other lubricants in conjunction with WD-40, they often avoid having it get gummy or turn into varnish.
So if someone sprays their gun down with WD-40 and they wipe it, and then they put some Break Free CLP or something on the rails or in some places, that oil tends to migrate around and it seems to prevent the WD-40 from getting gummy. Or maybe they spray with WD-40, but as a final step they wipe the gun down with RemOil or something.
There are plenty of people who use WD-40 and they never experience a problem with it.
I'm not saying it's a good lubricant, I actually switched to Lubriplate mentioned in Grant's article and I think it works great.