Or vice-versa. As best as I can tell, the impact driver is very good for most any type of screw going through wood. When would you use a drill (with screw bit) instead?
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.
I started using a DeWalt compact impact driver a couple of years ago, and will never go back to a regular drill/driver again, with a few exceptions. The impact driver is superior for most types of screws in hard wood, decking and sheathing. It is also great for self tapping and self drilling screws. It rarely strips the head of the screw as long as you keep the angle right and it sets the head well.
Some applications not well suited for an impact drill are drywall, fragile materials like thin woods, veneers, plastics, acrylics etc and on screws like machine screws in threaded metal.
When I can, I use a regular drill because it's quieter. Saves my hearing.
I also like the regular drill when precision is required. My drill has a low speed setting. Combined with gentle trigger pressure, I can get the screw to just the right depth. For example, when using pole barn screws w/ neoprene washers on metal roofing or siding.
In soft woods, like cedar, an impact driver in novice hands can do a lot of damage - the screw can go very deep in to the wood.
My new impact driver has 3 speed/torque settings and is also trigger sensitive, if I had to choose between a drill and an impact driver I'd take the impact driver any day because you can get drill bits for them for drilling also. They go OK at doing a drill's job but a drill will never do an impact driver's job: it will burn out. And a normal drill in novice hands is capable of a lot of destruction also: anything from over tightening to broken wrists, an impact driver will never break your wrist. That goes for any power tool though: they are all dangerous.
- When you don't have a drill around.
- When you do have a drill around but you have it ocuppied in some thing else. 2a. When you are using the drill for drilling, you can use the impact driver for driving screws; that way, you don't have to keep switching between drill bit and screw bit on your drill which can be tedious.
- Don't go crazy as to which one to use - just grab the one closest to you and after a little while, you will have your own personal favorite system all worked out.