From a student's perspective, I find it incredibly useful. I use a tablet PC as my main computer at school for note taking, writing on profs' slides, etc. I use the iPad for:
1) I downloaded a bunch of textbooks and loaded them onto the iPad, which saved me pretty much the cost of the thing.
2) I use it as a 2nd display when studying, so I can load up PDFs or PPTs on it to scroll through. With 4.2 installed, the ability to flip back and forth between things (you know, multitasking!) is very helpful.
3) I keep it with me pretty much everywhere as the Dropbox app has all of my work sync'd so I can quickly take some notes, or just read up on things.
4) It's great for media consumption... I have a whole bunch of Med blogs that I read compiled into a Twitter list, and then Flipboard brings down all of the links these guys post into a newspaper-style feed. Keeps my Google Reader free for other stuff. Also, TED.com vids when I have spare time. Enough said.
Having used one for a couple of months I would definitely say it's not for everyone but if you're a heavy "consumer" of media, it's an excellent device.