I use Quicksilver all the time. Despite its bugs, I prefer it to any of the other launcher type programs that exist. Nevermind the fact that it is more powerful than most of those other programs (Alfred, LaunchBar, etc). The single feature that got me hooked is the ability to bring up the Quicksilver prompt with a single keypress. I have it set up to use the command key.
Other tools like Alfred won't do this (though it does have the command key double tap).
Right around a year ago I started experiencing issues with single modifier activation not working occasionally. At first I didn't know if it was Quicksilver being buggy or what. Eventually I noticed that if my cursor was inside a password field single modifier activation would not work. This is apparently a restriction of the operating system, that doesn't allow Quicksilver (or other programs like it) to listen to text entry on secure fields.
I also noticed that when I was in Terminal.app single modifier activation would not work, which was strange because it had worked in the past (on 10.5 for example). As a stopgap measure I started using command+space, which seemed to work all the time.
Using that new keybinding was annoying enough that I switched my terminal and started using iTerm2.app. It did not have the same problem as Terminal.app oddly enough. I finally figured out what Terminal.app's problem was.
This setting essentially makes all text input in Terminal.app act like a password field having the same single modifier activation problem as above. Turning Secure Keyboard Entry off fixed the problem.