What is an Impact Driver?
This is the perfect tool if you are going to drive a lot of screws, say build a deck or drywall a room. An impact driver will greatly speed up a large construction project. But they are not for everyone. There is a lot of confusion over the new impact drivers. These cordless tools are not for removing nuts and bolts, but a tool for speeding up your home remodeling and construction projects.
Milwaukee Cordless
Basically they are high speed screwdrivers or drill/drivers that give the screw a little "tap" as it is being set. They allow you to quickly and easily screw in even the longest screw.
Impact drivers are not impact wrenches. Impact wrenches are the noise you hear from the garage when you have you car repaired and mechanics typically use them to quickly loosen and tighten large bolts and nuts.
Another confusion for you when searching on net is the original impact drivers are manual. You hit them with a hammer to loosen stuck nuts and bolts.
Impact drivers are rated in inch/pounds. Besides the inch/pound rating some manufactures also list the RPM and beats per minute.
Do you need an Impact Driver?
Impact drivers or impact screwdrivers are not for everyone. They do take a little practice to use them consistently without stripping the screw, sinking the screw too far or bending the screw and not getting it set correctly. If you do buy one and haven't used one before, take some scrap wood and the screws you will be using and drive a few dozen to get used to how it works.
Remember, just like using your drill/driver, if you are screwing into trim, hardwoods or even thinner pieces of pine, always pilot drill the hole or you may end up splitting the wood. To make the screw go into hardwoods easier, my secret is to this is to put a little bar soap on the threads. I keep a bar of Ivory soap right with my assortment of screws.
A cordless impact driver works really well for sheet metal construction like installing a tin roof, drywall on metal studs, and metal barn siding. Most are powerful enough to drive lag bolts and sensitive enough to drive sheet metal screws into duct work. In combination with your drill/driver these impact drivers will greatly speed up your home remodeling tasks.
Reviews
I have had a chance to use or demo quite a few of these wonderful tools in the last few months. These included drivers by Bosch, Craftsman, Hitachi, Makita, Milwaukee, Dewalt, and the new Panasonic.
Of course the Panasonic is the best, but it also is the most expensive. The three speeds make this the most versatile. The Craftsman Professional was the hardest for me to use. I stripped the heads out of a lot of screws trying to drive them into pine. It also broke 1/4 inch stove bolts before I could get my finger off the trigger. The Craftsman C3 19.2 volt lithium and the Milwaukee Lithium were good tools at a good value for me. They are available by themselves or in a drill/driver/battery combination kit.
The Craftsman NEXTEC was the least powerful, but it is intended for the beginning homeowner so I really can't compare it to the rest. If you already have a NEXTEC auto hammer or multi-tool buy this one because the batteries are interchangeable, otherwise buy the 19.2 Craftsman or the Milwaukee.
The rest I tried all work about the same so if you like Makita, buy a Makita.
Unless you have an existing drill/driver that still has good battery packs (say, buy an 18v Dewalt impact driver if you have a good 18 volt Dewalt Drill) I would strongly suggest going with the impact drivers that have the new lithium ion batteries. These impact drivers are much lighter in weight than Ni-cad powered ones and recharge quickly.
The best deal currently is the Milwaukee 2691-22 Combo kit. I know hundreds (yes hundreds) of contractors that use this drill and they all like it for the weight/power/battery ratio.