Impact drivers and shredded screw heads

If you can use a drill with a screwdriver bit to drive a screw, then most of the time, use it.

Here's when you might be smarter to use a screwdriver:

1. The screw is going into a pre-tapped (threaded) hole in metal, or plastic, or any soft material.
   Always at least start such screws by hand -- so that you can avoid cross-threading them.

2. Putting screws into delicate assemblies: eyeglasses, electronics, medical prosthetics, etc.

3. When working in cramped spaces, a drill/driver often can't fit. Screwdrivers come in flexible, stubby, right-angle, etc. varieties for such occasions.

4. For emergencies or odd jobs. Multi-bit screwdriver tools are dirt-cheap now. I have one in all my vehicles. I keep one in my briefcase, one at my grandmother's house, etc. You'd be surprised how often something needs to get screwed ;) and I certainly don't have dozens of cordless drills and chargers.
   

5. When the environment is likely to damage an electric device -- working in the rain, or in the mud under a house, or in a very gritty environment, etc. (Note that this is one more reason to make employees buy their own tools.)

6. When working with a material where finish is important. A driver is much more likely to strip the screw interface, or to slip and mar the material being screwed, or to drive the screw too deep and crack or warp the surface of the material.

7. When driving slot-head screws. These are notoriously hard to drive with a drill; the bit flies out of the slot too easily.

8. When a precise torque is needed. (NEVER trust the torque setting on a motorized driver unless it is one of the industrial models made for that purpose and it has been recently calibrated.)

9. When the screw itself is delicate, such as plastic screws, soft metal screws, threaded wooden dowels, etc.

10. When working around volatile chemicals or in an explosive environment.

11. When working in a clean-room environment, screwdrivers are much less likely to contaminate and are easier to sanitize/sterilize.

12. When chemical or radiological contamination is an issue. A plain screwdriver is much easier to decontaminate.

13. When the power has failed and/or the battery is dead (batteries also stop taking a charge after a while and need to be replaced). Thanks to ratchet freak for the reminder.

14. When you want a nice vodka "kick" to your orange juice.

Drills and impact drivers are similar in that they're both rotating power tools and they have some overlap in usage, but they work differently and have different strengths.

Drills apply a constant torque and tend to have good control across a range of speeds and clutch settings. They're good for work that requires care or precision, like drilling a hole or driving a a smaller screw.

Impact drivers provide much higher torque but in rapid bursts–very much like a hammer provides bursts of linear force. This makes them good for heavy-duty jobs like driving long lag bolts (you can even take the lug nuts off your car wheels with most impact drivers), but the banging action makes them a little bit difficult to control. They're also not good for driving into something very soft–between the hammer strikes there's no torque applied and the bit can actually rotate backwards. You can see this by holding the chuck with your hands while the driver is on; you'll feel the pulsing of the strikes but chuck won't actually rotate. Finally, some impact drivers don't have a clutch, which means you need to be very careful if you're driving something that could be over-tightened or broken by the high torque.

Besides the higher torque, impact drivers have another huge advantage in that you don't have to hold them tightly to provide the torque. The momentum of the rotating striker inside is what provides the rotation force, so even when you're driving something very tough it's quite easy to hold the tool. Drills on the other hand provide a constant torque and require the user to provide the reaction and prevent the tool from spinning.

If you have to pick a single all-purpose tool for general DIY use, a drill is much easier to control and is useful in a wider range of situations.