I'm constantly amazed by how Quicksilver becomes an unconscious part of your interaction with a computer. Take today, for example. I had to send some information by internal mail to one of the administrators in our department. I had a one of those all-too-frequent mental blackouts, and could remember her first name but not her surname. Without even thinking, I turned to my keyboard, hit Cmd-space to activate Quicksilver, typed the first few letters of her name, looked at the result (her card in Address Book), and thought "Oh, yes, that's it". I hit escape to dismiss the result (since I didn't actually need to open her card in Address Book) and turned back to the envelope to address it.
It was only while I was writing that I realised how easy it all was. Now if I could only activate the same sequence in my head when talking to people in person so that I could remember their names, I'd be a happy person.