With Tiger, I was a rabid Quicksilver fan. I tended to use it almost exclusively as an application launcher and to add todos to iGTD. Every now and then, I’d use it to delete a file or something, but it was almost always just launch or todo. I did also use it to look up contact information, which is something you can also do with Spotlight.
With Leopard, Spotlight works a lot better, and is very quick as a program launcher. Its calculator feature works nicely, and I’ve used that a couple of times. I’m trying out Mail’s new todos feature, and I’m not completely sold on that yet. Even if I stick with iGTD, though, iGTD has its own hotkey-to-add a todo feature. No need for Quicksilver just for that single feature.
Translucent menu bar aside, Leopard has been a nice upgrade overall.