Developer tools are for, um... developers. If you don't know why you need them, then you don't need them. They don't add "secret speed boosts", they're just applications for developing new applications.
iDisk is way faster on X than 9, but is still unacceptably slow. There's a good 5 to 10 seconds of waiting for any folder to show its contents, and then resizing the window might refresh the contents and block again. Argh.
What's really sad is that Windows 2000 can access my iDisk way faster...
While I'm glad that Apple made any attempt at full keyboard control, the results are less than stellar. The keyboard can be a faster alternative to the mouse in many situations, but X's implementation requires you to use the arrows to linearly traverse over all the menu items until you get to...