Disclaimer: I own every fucking Apple product made in the last 10 years. And 2 of some of them.
I use my iPad2 all day long - listening to music, playing games, taking notes, managing my OmniFocus todo lists, reading Slashdot.. you know, the usual. I have often thought "Could this replace my Macbook Pro?", and the answer is always "no!".
It isn't fast enough. It doesn't have tab from field to field, even with the external keyboard. While the apps that it does run are nice, they are not nearly as full featured as their "real" Mac based counterparts. There is no file system, so you can't manage your documents in a hierarchy like you can on a "real" Mac. Apparently this is what Apple wants to kill anyways... who needs folders, right? Well, I do. I like folders, and the iPad doesn't have them. There is no external storage, other than a very awkward (not to mention slow) interface to iDisk and/or Dropbox. And finally, there is no way to use my Magic Trackpad with the device... I constantly have to reach up and touch the screen when I am using the external keyboard.. and you know, after 15 minutes, my arm feels like it wants to fall off. ;-)
And there's one more thing... I can't install apps other than what Apple says is ok to install on the iPad. Until they give me full control over the device, it'll just be a toy that is good at managing todo lists and playing games.