For situations where the laptop (MBP 17") is too cumbersome and the iPhone is too cramped.I've been reading the iPad forums with interest - so many people heavily against and so many for.
Personally, I, like many, am incredibly disappointed. I desperately wanted a new toy to get my teeth into, but with my iPhone in my pocket and my MBP in my bag, I just don't see the point of the iPad.
So why are you going to buy one? I want to be convinced and would love to hear other people's reasons.
For instance, I probably spend a couple of hours a day on the bed or the couch surfing, mailing and watching movies. I find setting up the laptop to be too much of an, err, "event"... you have to get the mouse and plug in the power brick and get the laptop rest to protect whatever is underneath the laptop from the heat... I want something I can just grab and use, like the iPhone, but I hate surfing on that little thing. The scrolling, the zooming, the accidental clicking... it's the best surfing experience that a phone can offer, but why settle for the lesser of many evils? And don't get me started on writing emails on the puny onscreen keyboard.
There are also situations where you want to show someone something, like a picture or YouTube clip or a web page. Let's say I'll spot something and I want to show it to my girlfriend who is in another room. If I'm on the laptop I'll either yell "hey come here and check this out!" or pick up the laptop, walk over there and somehow awkwardly balance it in the air while I operate it. If I'm on the iPhone, I'll walk over and hold up the phone while she leans forward and squints to look at some tiny pic or Youtube clip.
The iPad appears ideal for both the "grab something surf-capable and use it in a hassle free way" situations and the "OMG check this out!" situations, and I'm willing to spend a few hundred on that. I wasn't expecting the iPad to be the second coming of Jesus or revolutionary in any way, it turned out to be pretty much what I wanted, with no positive or negative surprises attached.