Really, there are virtually no exceptions. The most popular is lighting control which can be very economical using Powerline Carrier Technology which uses existing wiring to communicate and control lights and appliances. The most cost effective is climate control of the HVAC system as the energy management functions provide comfort while saving energy costs. The most fun depends on personal habits and interests of the homeowner. For some it is audio distribution, others it is the ability to control anything from the Internet, and for others it is just showing the cool stuff to visitors.
Interfaces are many and depend on the desires of the homeowner.
Most prefer some sort of console or keypad strategically located by the front door, or masterbedroom, or a portable wireless version. Others want more interaction with the PC being most common for greater ease and control for schedule and programming modifications. The magic of any Home Automation system is the 'set and forget' automation which fits the lifestyle of the homeowner and family. Whether keypad, PC, wireless remote, telephone or dedicated switch interface all provide interaction with the automation system. The differences between systems are so broad that this question is difficult to answer in it's current form, however there are two basic paths to follow.