"Seat belts, pff! They kill more people than they save!"
"That's not true, you're thinking of airbags!
- Homer and Lisa Simpson

A nice idea in theory; you're in a crash, and before your head snaps forward on your neck, a nice soft bag comes up to gently cushion your skull, then disappears immediately so you can see to still control the car.

Airbags deploy in under a quarter of a second, and when used in conjunction with a seat belt they can be effective in staving off whiplash and other head injuries for the average person. Note I said 'average person'. Most people are not average, and in cases where the driver (or passenger) is shorter than what the engineers of the system have decided, fatalities can occur from airbag deployment. Of the deaths that have been attributed to airbags, most of them were short adults and children.

The airbag is usually activated when crashing at speeds of 25km/h or over, and should only activate in front or rear collisions (A bag inflating at speeds toward your face is of no use if your car is hit side on). Exceptions to this are cars produced by companies who consider themselves to be formost experts in safety; like Volvo, whose newer cars have over twenty airbags for all passengers. This is just stupid, and honestly, they should spend more time making a more controllable car than filling it with gimmicky safety features.

Airbags are designed to deflate immediately (If you want a nice, air filled pillow, get some Fill-Air), the theory being that after the initial impact, then car will most likely still be moving, and the driver will be able to control the car better (to avoid a worse accident) if he can see. Good idea? Yes, but let's examine what happens: An airbag inflates at around 400km/h. This is fast enough to break your nose, cause other serious facial injuries, or even cause death. All of which has been known to happen. There is usually also a lot of dust released with the airbag, due to the installation process and the time between the car being made and the time the airbag is used. Also depending on how the driver's holding the wheel his hands/arms may get broken. This is worse than a punch in the face, so do you think you'd still be in a good state to steer a car and make decisions about the road? I wouldn't.

Bullbars have been known to cause adverse airbag behaviour, upsetting the timing device and causing the airbag to operate either too late or too early. I laugh at the people in their Toorak Tractors.

Summing up, in no way would I actively seek out a car with airbags. If I did buy a car with them, I would probably ask the dealership if they could be removed.

Note: No legislature exists at the moment decreeing that airbags are standard (although some car manufacturers insist on making them standard among some of their models). Passive safety features are standard. Airbags are classified as active.
For those chemically minded, here is the three step chemical reaction that inflates airbags, gathered from http://www.madsci.org/posts/archives/aug99/934263277.Ch.r.html

This initial reaction forms sodium and hot nitrogen gas which inflates the airbag.
2 NaN3 —> 2 Na + 3 N2

The sodium byproduct of the first reaction and the potassium nitrate generate additional nitrogen in the secondary reaction.

10 Na + 2 KNO3 —> K2O + 5 Na2O + N2

And finally the previous two reactions leave potassium oxide and sodium oxide to react with the third component of the mixture, silicon dioxide, forming alkaline silicate "glass".

K2O + Na2O + SiO2 —> alkaline silicate

As you can see, the reactions in steps 1 and 2 release a great deal of nitrogen gas. It is this hot nitrogen gas that fills the airbag. The potentially harmful sodium created in step 1 combines with potassium nitrate in step 2 to produce more nitrogen, potassium oxide, and sodium oxide. The final result is nitrogen gas and alkaline silicate powder. The sodium produced in step 1 may also react with moisture, temporarily forming sodium hydroxide. Because these reactions occur so rapidly, the multiple steps in the reaction are in reality occurring simultaneously.