I received this tool in the combo kit with the compact drill/driver, which I bought from Amazon largely for the drill.
I had originally bought the cheap Porter Cable 4 part Combo Kit from Lowes (Drill, Reciprocating Saw, Circular Saw, Light), only to have the forward/reverse control on that drill break within a year. I could still use it theoretically, but only for removing screws. Since I already had the batteries with my kit, I went to Lowes and purchased the bare tool Porter Cable Impact Driver ($40 at the time), and ended up using that for months for most of my work (hanging sheetrock, HVAC systems, etc). Eventually, it became more of a pain to pre-drill holes, or do anything with a bit that needed a chuck to fit onto it.
I made a couple trips to Home Depot and some other Hardware stores, and tried out the tools. I knew Milwaukee had one of the best reputations, and wanted something that was ergonomic, and used the Lithium-Ion batteries, and luckily the Milwaukee Compact Drill/Driver seemed like the most comfortable tool there, and it fit what I wanted. I went onto Amazon and bought the combo kit that came with the compact drill/driver and this impact driver, two compact batteries, one pocket clip, and the charger, which was only $200, compared to the same price for just the drill/driver at Home Depot.
The first job I had when I got these tools was replacing some metal roofing, and either tool worked fine, but I can't emphasize enough how important the pocket clip is if you're going to be doing roofing, framing, or sheetrock. It's also useful when you're putting up cabinets and need somewhere to place your drill. Unfortunately my kit only came with the one, so I chose to put it on the drill/driver, expecting I'd be using it more. Now that I've had the kit for almost 5 months, I have to say that was a premature decision. I'll be purchasing another clip whenever I can find one (I didn't see them at Home Depot) for my Impact Driver, because that's become my favorite tool to use for most applications.
I've used it for driving deck screws through TREX, and it worked phenomenally. I've used it with a socket attachment with 7-9 in. lags when I was attaching 4"x4" posts to beams on a large commercial deck, and it worked splendidly. I was going to be stuck with my socket where I was, and started off using it before trying the impact driver, and was pleasantly surprised. It did start to slow down when the lag got deeper, but it was still able to keep driving the lag while sinking the washer fairly deep into the treated 4"x4".
The compact size of the impact driver also makes it useful for crawlspace work. I use it often when working on HVAC systems and need to drive sheet metal screws into pipe. Perhaps due to the short length of the tool, it is a lot easier to control, and slipping has become a much less common occurrence. It's much easier to apply force in the direction that you are trying to with this tool versus a normal drill.
The only time when I don't use this impact driver is when I actually need to drill something, or I need more reach, in which case I use the drill with a finder driver, or a larger bit extension (tightening screws on the ceiling that didn't sink well enough for mudding, etc.).
Since I've gotten this kit, my boss rarely ever uses his Ridgid NiCad Hammer Drill/Driver, opting instead to borrow one of my tools for most applications. They're a great buy, but for most things, the Impact Driver is the most useful to have. It's more compact than the drill, and it's far more powerful. If you're using a good brand (DeWalt, Makita, etc), there's probably no financially good reason to switch over to this impact driver, but for those making the decision, I have had no regrets about owning this tool, and the exclusive battery meter on the Milwaukee M18 batteries just sweetens the deal.