Impact drivers provide about 3X more peak torque than a similar regular drill. The drawbacks are they are (slightly) heavier, (much) noisier, and slower than normal drills. When deciding whether you want/need an impact driver, it helps to know how they work.
Impact drivers work by storing energy in a spring and then releasing that energy. That means they pulse rather than providing smooth even power. But the storing and releasing of energy happens so quickly that you won't notice a pulse. What you will notice is driving is much slower, but also more powerful. Actually you won't notice the power unless you're doing the same task with a non-impact driver.
And that's probably the best thing about an impact driver. You can drive a 3/8" lag screw and it feels like you're not doing much work. If you drive that same screw with a regular drill it'll take your arm off if you're not careful. Driving several tough screws can wear you out with a regular drill, but with an impact driver you'll hardly notice.
What you will notice is how much longer it takes. And the noise. They can be quite loud. But these aren't bad trade-offs for the extra torque the drill provides and the decreased muscle you have to provide.
The last drawback is that impact drivers aren't good for drilling holes. If you own one you'll also need a regular driver. That's why it's a good idea to get one in a combo pack when you purchase a regular drill. Since they'll use the same batteries you won't have to pay much more to get the impact driver along with the regular drill.