A little while ago I was singing the praises of the battery drill and now I'd like to write about its close cousin, the impact driver. Again, until relatively recently, I had not used one of these things and I didn't fully understand how they differed from their bigger relative, the battery drill. The main difference is that a dedicated impact driver doesn't have an adjustable chuck or an adjustable torque setting. Instead, the impact driver has a collet that accepts hex driver bits which really describes its sole purpose, to drive things in (or out, depending on the circumstance).
As its name implies, the impact driver drives with impact which is delivered by an internal mechanism which pulses the rotational effort in a concussive manner. The result is that the impact driver overcomes any frictional resistance between, say, a screw and the material that it is being driven into. All good except that this capability, although delivering a much higher work rate (not quite as fast as a nail gun but not far away) has its downsides - specifically that pilot holes and countersinking are, in some situations, things of the past.
The images above show how the impact driver can effect finish in that the impact driver can drive a pretty beefy screw straight into ply over battens and then through the ply and out the other side, if you let it. A drill driver, with its adjustable torque setting, can be used as a rather more precise tool but it doesn't have the out and out power of the impact driver.
But, in some situations, you can use the drill/driver as an ersatz impact driver. Recently I had to remove a screw holding the heat resistant glass plate across the front of our wood burner. As anyone who has tried this will know, such screws can be somewhat difficult to remove and this was a case in point. However, using my drill/driver in reverse setting and slowly increasing the torque setting, the drill/driver acted as an impact driver and managed to remove the screw in question.
So, its all about horse for courses and the drill/driver is still the most flexible tool but, for raw power, you can't beat the impact driver.