Running on a brand new Dell Inspiron 9400 with 2GB of RAM and a dual-core processor my experience so far has been that Outlook 2007 is extremely slow.  Clicking from one inbox item to the next takes up to 10 seconds to display the message content.  Clicking from one calendar item to the next takes up to 5 seconds.
Then, inexplicably about a half hour after you’ve had Outlook open, those delays sometimes go away to where there’s absolutely no delay at all. 
2007-02-11 – REVISED: OK, Outlook isn’t that slow.  Yes, there are a few things that take longer to do, but I think most of my initial performance delay was due to my 6GB mailbox being indexed by the Windows Desktop Search and X1 search engines.
In general the new Outlook UI is great.  There are many usable improvements, but not so many that this is a must have vs. Outlook 2003.
The most annoying thing for people like me who use Outlook to log their time is that unlike good old Outlook 98, reminders are still displayed for appointments you create in the past.  In Outlook 98 and before if you create an appointment in the past no reminder was set.  If you created it in the future, a reminder was set.  Since Outlook 2000 that functionality was changed so that if you have the default reminders feature turned on, the reminder is ALWAYS set even for appointments you create in the past when logging your time, so you get an annoying prompt to clear the reminder after creating every entry.
Other Office 2007 features are great.  The conditional formatting rules in Excel are almost worth the upgrade price alone for anyone doing analysis of columns of numbers. 
2007-02-11 – REVISED: I keep finding things in Outlook that make it a great upgrade.  There’s a lot more flexibility in how you show information — People who use tasks to manage their time will appreciate the upgrade.