I went into an Apple store to buy an iPad and walked out with an 11" Macbook Air. They are more capable, with a built in full keyboard and everything you can do on a full sized desktop computer (even can run Windows), but without an optical drive. On the other hand, we need another laptop and now we're looking again at the iPad. Rumors are that MS is coming out with a version of Office, which would be a game-changer for me as far as being able to directly work on Word or Excel files. Ability to print is much improved with iPads (and even iPhones), and they are indeed lighter and cheaper. The rumored $800 Air would make that a more difficult decision again, because it would not be that much more than a top of the line iPad. But they are still apples and oranges that both happen to be cool and do many of the same things, just differently. Whichever one you buy, you may wish you had the other one. The right answer if you have the money is to just get both;)
PS- If you're planning on sharing your device with one or more other people, a Macbook (Air or Pro) is a better choice, as you can have multiple user accounts. An iPad is really a personal device, more like an iPhone or iPod Touch that way.