I just switched back to Quicksilver after trying Alfred for a bit. The basic reason was that Quicksilver's search is better and more easily customised: it finds the things I'm looking for, and given that the whole point of using such things is to improve one's productivity (by cutting out the whole "Open Finder, dig down through the file hierarchy, finally manage to open the file, directory or application being searched for" process), it doesn't really matter to me how well its competitors do in other areas.