I loved QuickSilver until I traced a system-wide drag-and-drop problem back to it and regretfully uninstalled it in favor of LaunchBar (which is not free). This was around the time the original developer announced he was basically ceasing work on it.

I'm glad development has resumed for QuickSilver, but I'll wait and see before giving up the reliably updated LaunchBar.

It went through a bad patch a few years ago. For more than a year now it has been rock solid for me.

It seems to me that the value in Quicksilver is the advanced operators you can do - for me I only ever used it for launching/file searching and Alfred fills the hole nicely - I don't intend to switch back.

Quicksilver is pretty much what every other launcher on the Mac has been based on, but for free. Alfred is a nice product and basic launching functionality is free, but most of the advanced features that are free in Quicksilver cost £12 (about $19) to add.

I've been running the previous version of Quicksilver on Snow Leopard and Lion, and it's been very solid; the new version should be even better.