To rate performance under major strain, I sank two 1/2-by-6-inch screws with each tool into the simulated ledger assembly through predrilled 3/8-inch holes, timed how long the tools took to seat each screw, and recorded the average.
The king of the hill was Hitachi's 14.4-volt model at an average of 14.5 seconds. It felt powerful and drove the lag down hard. It also had the single best time: 13 seconds. Next came the 14.4-volt Milwaukee at 16 seconds with consistently powerful drives; the 18-volt DeWalt at 17.5 seconds; and the 12-volt Hitachi and 14.4-volt DeWalt, both at 18 seconds. These last two tools felt strong up until the very end when they strained, but the work they accomplished was reasonable. The 12-volt Panasonic EY7201GQW was next, sinking the screws in 20 seconds, but was definitely working hard to seat them. Then, the 14.4-volt Makita and 12-volt Panasonic EY6506NQKW both sank the screws in 21 seconds. Makita's 12-volt tool took 24 seconds.
Screwdriving. Since my arm would fall off long before I could wear all these tools down driving 3-inch deck screws, I chose 4-inch-long Timberlock screws to see how long the batteries would last when pushed to the screw-turning max, counting how many each tool twisted in before the battery showed a noticeable slowdown.
Even driving these thicker, heavier screws, my arm almost did fall off trying to run the Milwaukee 14.4-volt tool and 18-volt DeWalt dry. The 14.4-volt Milwaukee outran the group, sending 46 Timberlocks crunching into the stock. Close behind, DeWalt's 18-volt tool knocked down 42. Next came the 12-volt Panasonic EY7201GQW with 37, Hitachi's 14.4-volt with 36, and the Panasonic EY6506NQKW with 32. The Makita 12-volt unit had 30 solid sets.
Makita's 14.4-volt tool was next with 28 Timberlocks, followed by the 12-volt Hitachi with 24 screws nicely set. The 14.4-volt DeWalt solidly sank 21.
Fit & Finish
Rubber handles and bonnets make it easy to get a solid, comfortable hold on the tools.
Balance and Feel. If an 18-volt cordless drill is a .44 magnum, then an impact driver is James Bond's Walter PPK–snug, small, and low-profile, but equally effective. The first thing I noticed when I picked up the 12-volt Hitachi and Panasonic EY6506NQKW is that they nestled well in my hand. The handles are easy to hold and the tools are easy to manipulate, whether driving boxes of deck screws or craning my arm to start a screw in the top hinge of a door. The rubber handles and bonnets are excellent spots for my opposite hand to guide the tools. The triggers are also nice on both units, with no pinch points. Next, I found both DeWalt tools very com fortable. I like the rounded rubber triggers and handles as well as the rubber bonnets covering the back of the tools.
Milwaukee's kit box is terrific. It's tough, the bits stay put, and it has a place for the charger cord.
The rest of the tools are all nicely balanced and comfortable enough for all-day use. However, while I found the Panasonic EY7201GQW comfortable, my hands are small and I think its handle would be snug for workers with larger hands. In contrast, both Makitas' handles are thick (to accommodate their larger battery stem) and feel like they'd be more at home on a larger tool–or more comfortable for workers with larger hands than mine. The 14.4-volt Hitachi and Milwaukee also fit my hand fine, but since they're the largest tools in the bunch, these models feel more like drill/drivers than impact drivers.
Bit Exchange. The whole world should be on the 1/4-inch hex drive system. With all due respect to keyless chucks, they can't come close to a hex drive's plush operation–bits pop in and out without a hiccup and once they're locked in, they don't slip or fall out. Ever. Plus, a hex drive is about one-tenth the size of a keyless chuck with fewer parts to break or wear out. All the drives worked perfectly. Forced to pick, though, Makita's is best: Its knurled collar is totally non-slip and easy to grab, even with gloves on. I really liked it for hanging doors, where I'd switch between drill and driver bits frequently.
Battery Exchange. Even with my small hands, the battery release pinch tabs on both of the DeWalt models and both Makitas are easy to reach, and they release the batteries effortlessly. Next, I like Milwaukee's slide-on battery. It takes a few tries to get used to its different design, but it works great. Also, this battery is reversible, which is a nice feature. Both Hitachi units have fine releases. The Panasonics' are a little awkward because I can't reach them from the bottom of the tool; instead, I have to get at them from the front, which isn't my natural instinct.
Toolbox. A good toolbox is important to me for storing and transporting tools and bits. Milwaukee's is best, with a smart layout and a great compartment for extra bits. The Panasonic boxes are also tough. The rest of the boxes are serviceable, but don't appear as tough.