For me it's simple, it's about battery life and convenience.
The iPad gets used constantly throughout the day, and the MacBook air gets used when I need to do something I can't do on the iPad or that will take more time to do. I'm talking specifically about my working day here.
If i need to write something lengthy, that requires simple text editing, I'll whip out an external keyboard and stick with the iPad, anything more complex and it's the Air.
One thing i do find amusing though, is on occasion I have friends who complain that the keyboard on the iPad is too slow for them, and they're much faster on a laptop. I asked the last person who said this to me to do a speed test on his laptop. He hit 40wpm, because he doesn't touch type. I hit 42wpm on the iPad, I'm circa 80wpm on a full sized keyboard.
i don't think the iPad is painfully slow for text input, it's sure as hell much faster for me than using a smart phone to reply to emails that's for sure.
The other thing i use the iPad for is presenting, I work in sales. I use it to display brochures, PDF Documents, data on spreadsheets, and I always run my Keynote presentations from my iPad. Add into the mix Google maps, for planning journeys and suggesting meet up points, Webex for training courses, conference calls, and meetings, browsing customer web sites, checking the companies database/ CRM tool, inputting expenses, To do list, etc etc. Honestly i could ramble on loads.
Basically it's a hell of a lot more convenient than using a laptop, both in terms of form factor (using it while walking or just standing up for example), and much faster than plugging a 3G dongle in and having to connect. I do wish Apple would have put a sim tray in the Air. And the battery life is paramount for me, I've never got it past 18% by the end of a working day.
If i need to do a quote though, or multi task, i.e. heavier work, i'll use the MacBook Air
As a business tool, i find the iPad invaluable. For home use, I never use it. I always use the MacBook Air, which sounds like the reverse of quite a few posters on this thread