In general, I used to think that I could do everything on a laptop that I could do on an iPad. The advantage of the iPad lies purely in the form factor: portability, ease of use. While I can read a book with a MacBook air, I find it more convenient on an iPad. While I could read sheet music on a MacBook, I find it more convenient on an iPad. While I could use a laptop on an airplane, I found the iPad more convenient. While my MacBook air is light and portable, the iPad is even more so.
More recently, however, I continue to explore apps that allow me to do things on the iPad that I CANNOT do on the MacBook. These apps are generally related to remote control of other devices such TVs and other computer programs. While I am not yet happy with the TV remote apps to use one regurarly, I do use apps to remotely control ProPresentor, keynote, and powerpoint. These apps, in my mind, give the iPad (or, more generically, a tablet) a unique capability, I started to realize.
Of course, whether one values these features enought to spend $500 or more is a question that only can be answered by each person.