There's no question that a Windows PC or a MacBook is a better business system than an iPad. And as another poster pointed out, you can buy a lot of laptop for the price of an iPad these days.
But having said that, I've now replaced my travel netbook (an HP Mini) with an iPad 2. Why? One reason. The iPad combined with a portable bluetooth keyboard and an Origami Workstation stand/keboard case is far easier to use when crammed into a coach seat on a long flight than my netbook. And on a coast to coast flight with multiple layovers, I no longer have to worry about conserving the battery or carrying an extra one.
Truly serious work? I have a Dell Precision workstation for that. But the advantage of its 1920x1080 15.6" screen is negated on a long flight when the a**h**e in the seat in front of me decides he wants to recline his seat to the max. So I just leave the laptop in my carry-on bag and take it out when I'm in my hotel room or a client site.
All in all, the iPad saves me a pound or two in my briefcase compared to my netbook, gives me a better keyboard than the less than ideal netbook, lasts about twice as long (or more) on a charge, and gives me multiple options for its use when I'm confined to a tiny workspace on a plane.
Frankly, I've been surprised by how much I prefer the iPad to my netbook. It doesn't replace my quadcore laptop, but it has made the netbook virtually obsolete.