I’m one of the very many people who find Quicksilver to be the most valuable utility on their Mac, though I’m far from being a power-user. There are some people who start almost every activity with a Quicksilver keystroke, while I, for a long time, used it simply as a quick way to fire up apps that weren’t quite important enough to go in my Dock.
Great though it is, however, it’s far from being self-explanatory, especially for some of the more esoteric features! Fortunately, there are lots of tutorials out there from various enthusiasts to get you up and running. Some of the ones on Lifehacker.com are full of useful tips. You could do worse than starting here and following the links in the first paragraph. Even experienced users will probably learn something.
Or explore some of the Quicksilver-related screencasts on The Apple Blog or elsewhere. A Google search will find lots for you, and you’ll soon be on your way to guruhood.
As everybody says, once you get used to it being on your Mac, you’ll really miss it on machines which don’t have it. And the best news? It’s free!