I have been using my 3rd gen iPad as my primary "computer" since I sold my old 2008 MacBook Pro earlier this year... and it simply doesn't work as well as a full fledged operating system does. Don't get me wrong, I love my iPad, but I feel that aside from some limited edits on documents through the Pages app, it really can't replace a computer yet. With that said, I'm certain I need a mobile computer of some sort. I should point out that I do currently have a PC desktop running Windows 7 that I mostly use as a storage unit and blu ray/dvd player that is connected to my tv.
So what do I need a laptop for? I regularly (or at least used to prior to selling my old MB Pro) do research, work on papers/publications for journals, browse the web, edit photos, and also some hobbies including music composition and video editing. I've moved mostly to the cloud in a sense and as I mentioned have a 2TB external drive and my Windows desktop so storage on the Air isn't a major concern for me, nor is the lack of a disc drive. However, I am concerned that certain tasks won't work as well on the Air as they would on the Pro. Also, will the Air not be OS upgradeable in the future comparatively to the Pro?
Having an iPhone and iPad, I'd say I'm fairly locked in (and happy) with ecosystem, so I'd prefer sticking with Apple as switching to Windows 8 doesn't seem like an option despite it definitely having a "cool" factor to it now.