I have both and the number one problem I have with the iPad is that it overlaps too much with my iPhone. All of the key apps I use on the iOS platform are available for both devices, so it began to make less and less sense to carry my iPad around (much less get a separate data plan for it). When the new 11" MBA came out, I quickly realized that what I really wanted was something that was light/easily portable and full featured. I spent all this time looking for iOS apps that would turn the iPad into a full fledged computer when really all I needed was in fact a full fledged computer.
That said, I'm still hanging onto the iPad in part because I don't need the money that badly to have to sell it, but I use it a lot less and don't really miss it when it's not with me. I have no doubt about the potential of this form factor and I'm sure we'll see great advances in the next few years, but the current limitations are too much to keep it from being a big toy.