I can only give you my own experience, but I spend about two hours a day with the iPad. What I mostly use it for is what I call "casual computing" -- nothing too important. FaceTime, e-mail, surfing, reading and photography viewing, games and now (more and more) photo editing.
I really, REALLY enjoy the iPad, as it helps me separate my "work life" from my "leisure life," it has helped me to read more for pleasure, and it's powerful enough to take with me on trips instead of the MacBook all the time. I had until yesterday the iPad 1 but I sold it (for half what I paid for it, which is more than I expected) and bought the new one today. Still "restoring" it at the moment but I'm sure I will enjoy it even more, particularly for reading.
As I mentioned, I already have a "regular" Mac (a MacBook, soon to upgrade to a MacBook Pro) and I also have an iPhone 4 so there's a bit of overlap between the iPhone 4 and the iPad, but not enough to detract from the iPad. Now that I have the new iPad I expect to start using it more for TV/Movie viewing as well.