Now that I've had the tool for close to 3 years, I've found that indeed, it has far increased the speed at which I create and edit art. The keyboard shortcut (Shift-M) is now completely automatic, and using the tool requires absolutely no thought. It's not that Pathfinder was bad -- it's just extremely inefficient. I no longer need to move my eyes (or my cursor) away from the art I'm working on, and then figure out which icon in the Pathfinder panel I should click on. Even if I know what specific function I need, it still requires moving my focus away from my art. The Shape Builder is incredibly liberating.
If you're looking for some more information on how to use the Shape Builder, I've assembled a few resources for you:
Fridays with Mordy: Building Art in Illustrator. This is a 45min recording that discusses a variety of techniques for building artwork in Illustrator, and cover the use of the Shape Builder tool (Free).
Illustrator Insider Training: Drawing Without the Pen Tool. This is one of my lynda.com courses that focuses specifically on drawing vector artwork without having to rely on the Pen tool in Illustrator. It covers a variety of approaches and has a chapter dedicated to the use of the Shape Builder (Requires a lynda.com membership).
Here's an overview of the Shape Builder tool from Adobe TV (Free):
So if you aren't already making the most of the Shape Builder tool, 2013 is the year to make it so. Learn how it works, memorize the shortcuts (modifier keys enhance the functionality of the tool as you use it), and you'll be well on your way to a far more efficient drawing experience.
Feel free to share your own experiences with the Shape Builder tool in the comments below. Wishing all Illustrator users around the globe a wonderful and safe Holiday season and a Happy New Year!