Yes, they are very easy and efficient to use. I personally have a "Dewalt" tool, and it works great. Rechargeable, powerful, and saves so much time on all projects.
Yes. These are primarily designed for installing decking and other repetitive jobs. They are great for saving you time. The only drawback is that you are limited on the types of screws you can use in them. Another problem with them is the cost.
I actually have a screwdriving tool and it makes completing work around the house and backyard so much easier and helps me do the work much more efficiently!