I am using LaunchBar on my Mac, it is $20 for a single user, but it is worth for the cost, I see spotlight from Leopard can do the same kind of job as LaunchBar does, but I still choose LB on my everyday basis use because LaunchBar is more fast -- Actually I only use Mac at night after work.
Here is a video shows the basic usage of LaunchBar.
Here is my experience, basically I am using LB to launch applications, I can launch application from Finder Application directory, or click an application to run from the Dock but I don't want to a long list of applications in the dock, I want the dock to be simple. so I feel use LB is the best choice for me. The second thing with LB is browse the internet, I found LB remembers well bookmarks of Safari or Firefox, so if I wanna quick goes to the website I have to save the site to bookmarks of my Firefox as an example, otherwise LB will have no way to remember. But I am using delicous a lot, the hard thing for LB is it is not supported to the share society bookmarks, still did not try delicious2Safari (correct me if the name is not correct). There is some other beautiful functions by LB explained on the video, but I did not have a chance to use it.
At last, I think the experience will grows with using LB.