I used Quicksilver when it first came out. I didn’t really use many of the ‘advanced’ features, I just used it as an app launcher. The only non-app-launcher feature I used was the Play, Stop, Pause, etc. iTunes bits. (ok, they’re AppleScripts, so I guess technically I was only using it as an app launcher).
Anyway, I stopped using it when Spotlight became available, because it filled my needs without being an extra app to maintain. I’m a simple fellow, I don’t like lots of apps when I don’t need them.
I recently fired up QS again when I was procrastinating, and it’s won me over again. I’ll still probably not use the non-app-launcher features, but regardless, it’s damn quick. My Spotlight takes quite some time to search for apps, but Quicksilver loads stuff immediately.