The greatest problem with WD40 is that it attracts humidity. The alcohols in the substance are responsible for that. Use it as a lubricant, and you'll soon find that it promotes rust.

That said, it makes a wonderful degreaser. I wax my chains, and WD40 will cut through the wax buildup quickly and easily. Likewise, it will take grime and tar off of a frame or drivetrain, remove road tar from your automobile, possibly even degrease your cookware and limber up your arthritic joints.

Also... I've a sport motorbike, a Yamaha R6. I must lube the chain every 500mi or so, and upon doing this, I end up lubing the rest of the planet for days afterward. The rear wheel and bodywork become speckled with tenacious chain lube. WD40 gets it right off, and won't harm the paint. In fact, I've taken to wiping down the rear wheel and swingarm with WD BEFORE I lube the chain, to ease future cleanup.

DO NOT clean your tires with WD40! On a similar note, you do know better than to shine your tires with Armorall? You could get a firsthand lesson in elementary physics... friction, inertia, and the amazing abrasiveness of tarmac.