The DeWalt impact driver is
an awesome innovation in drill technology.
We've been using ours for a
few months now. I could tell you how the torque feature lets the drill
decide when to add more power or back off by making a loud clicking
sound, but the best way to appreciate what a great tool this is would
be to look at the reviews on Amazon. 111 five star reviews with only 1
one star review from a guy who had issues with how his driver smelled.
A good example is shelf
brackets. I was putting up a new shelf in the barn recently trying to
use the old Dewalt regular drill. The wood was hard enough to cause
problems. Either it would break off the head or stop gripping and begin
to strip the screw. When I finally walked over to get the impact drill
the difference was obvious.
If the wood is very hard, it is usually better to drill holes for the screws. Otherwise the wood may split.
That's why I bought a cordless drill with a removeable chuck. It has a hexagonal bit holder, and a chuck that just snaps on over that (even with a bit installed). It's very convenient; drill a hole, take off the chuck (quick-release), put in a screw.
Mine's a Black & Decker, but they don't seem to make them anymore. Maybe you can still find them on craiglist?
Here is one: http://www.amazon.com/DEWALT-DW0521-Connect-Impact-Conversion/dp/B000KKWLE8
There are better versions and cheaper also. I have a similar one and it is very useful.