In comparison to other platforms, installing software on OS X is a breeze. Usually, you just drag and drop the program from the disk image to its destination, and you're done. Even some complex programs can be a snap to install -- Office 2004, for instance, has its installer hidden in its code, and it's smart enough to run the first time you launch any Office app. So even though it installs stuff to a bunch of places, it's transparent to the user.
The third option is Apple's installer, which helps guide the user through the software installation process. The installer is the ideal solution for programs that need to install things in many spots, and require administrative access to do so. And while using the installer is still a very simple process, I still find it a frustrating process at times.
As an example, consider my recent installation of a new version of Snapz Pro X, the indispensible screen capture tool. Please note that this is not intended to be a slam on Apple's installer or Snapz Pro X (which I rely on every day!). Rather, it's just an example of how the process can be a bit frustrating and confusing, along with a couple of suggested improvements.