I love my iPad (I have the 32GB 3G model).
I use Keynote a lot (biochemistry slides), and other apps:
Epicurious is great (recipes on the fly - I bring my iPad with me to the grocery store, and check off the things I need on the ingredients list). I really love the built-in maps app and the directions/GPS features it has. The email app is great - I can go for a walk and send email in the middle of the woods.
The browser inside is great - I just used it to look up a rare battery that I needed inside Radio Shack.
I really like the books app, and I wish they would put the brightness slider in more apps. That would be great for late-night browsing.
I think everyone will have one sooner or later. It's just so practical for so many things.
When someone takes the trouble to make an app for something, it makes the experience much better than it is with a browser. I wish that someone would finally come up with a educational physics app (kind of like MasteringPhysics) for it.
I almost forgot - the $39 Apple case is definitely required - you can't take it anywhere without something to protect it.