I have been using Fast User Switching in Mac OS X. I have found that it is a great way to test new software, without worrying about the software corrupting my user files. I have created a test account, and I run the software with that user. I can easily switch back and forth between my account and the test account.
The only problem with Apple's implementation is that the user switch menu item is pretty large, as it uses the users full name as the menu title. WinSwitch is a better implementation of this menu with a lot more features. Not only does it let you change how the menu appears, but you can specify scripts or applications that get run when switching to or from a user.