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.
agreed; i've been runing quicksilver non-stop since before the developer passed the project on to others. for a time, it was sluggish and caused some problems. but i have no complaints to speak of about the last couple iterations.
i switched to quicksilver from launchbar, in spite of the fact that i'd paid good money for launchbar. it's not so much that something was wrong with launchbar; it's only that quicksilver was noticeably better in many respects (flexibility, esthetics, flow), imho.
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.