iCal
The short version: iCal 2 is a piece of junk. I admit that I haven't had time to do any real testing yet or report the bugs but at work where I had been happily using iCal 1.3 amidst a bunch of people on Entourage, iCal 2 seems to be completely broken. Clicking on events does not show anything in the info pane or unselect the previously selected event. Invitations come in all grayed out and show up in a "Notifications" bar (as well as showing how many notifications I have in the dock) but I can't do anything with them or make them go away. The following image shows what it looks like, in case anyone has some idea what the heck is going on.

Spotlight
At first I thought spotlight was going to replace Quicksilver / LaunchBar as an application launcher. However, it doesn't take long to figure out that isn't the case for several reasons:
- Spotlight is kinda slow.
- It doesn't learn from your behavior.
- Matches are continuously moved around in the list as they are found.
Where spotlight does work well is from the finder, where it sorts out matches by Folders, Images, HTML, Documents, etc. It also feels pretty fast when used this way. I got rid of the default Command + Space shortcut to the spotlight menu bar and went back to Quicksilver.
Slideshow
Slideshow is a much needed and unbelievably overdue feature, however the implementation is a little weak. It allows you to view a slideshow of images, rather than opening them in an application. The problem is that the only way to choose images for viewing is to browse into a directory, select the images, then right-click and choose slideshow. An alternative would be to do a spotlight search on the directory for something like ".jpg" then from the results a slideshow button is available. I really think you should be able to right-click on a directory and choose slideshow to see a slideshow of all the images in that directory. However don't expect to see that option, I submitted the feature request to Apple and here is what they had to say:
Engineering has determined that this issue behaves as intended based on the following information:
You might make the case for all kinds of things to be done with the contents of a folder, open all the images in a slideshow, open all the text documents in TextEdit, open all the files in their creator...but that contextual menu for folders would get long pretty quickly. It would be a convenient thing to be able to do, but it's not an action that's specific to a folder, which all the other contextual options are. You can however use Preview's Open dialog to choose a folder, and it will open all the images in the folder and every nested subfolder, then do a slideshow from that.
I obviously disagree. If a folder contains only images, then it should know that and give you the slideshow option. Also opening a few hundred images of several MB each in size that I've just downloaded from my camera is totally impractical and whenever I do so I usually have to force-quite Preview. One idea I have is to add this feature with Automator — it sounds possible.
Dashboard
Dashboard looks gimmicky but it's actually pretty useful. I've mapped it to a mouse button so it's nice to say, check the weather with the weather widget at the click of a button, since I can't really see out from my dingy cube. I haven't carefully checked to see how much resources its wasting, but I also haven't had any problems. The default stickies widget is completely broken and there is definitely some weird behavior (Check out the Daring Fireball Tiger Details List for examples). Anyway, I have built a widget to display my webcam (which I use to watch the dog) and I'll make available as soon as I add a couple things. It's turned out to be pretty convenient.
Summary
As far as speed goes, I really can't say that I've noticed any increase on either of my machines. For the most part it seems pretty much the same, occasionally some apps seem a bit faster and other ones seem a bit slower.
A couple of other things I've noticed that I hadn't seen anywhere else:
- Screen Capture (Command + Shift + 3 or 4) now saves the files to your desktop as PNG's instead of PDF's which is definitely an improvement.
- When using you Pictures directory for the Screen Saver, the images are now displayed entirely at random from all sub directories. Previously they were displayed in order, so that you always saw the same images over and over.
The Tiger Details list has been interesting to watch and I found the Dock Accepts Clippings via Drag and Drop and Keyboard Shortcuts in Dashboard both to be quite helpful. The one tool I most look forward to trying, but haven't had the time? Automator. So, got anything to add?