I've got an iMac as my main machine at home for work and play. When I hit Starbucks or feel like working from somewhere else in the house, I grab my MBP. I take the MBA when I head downtown to visit clients.
I keep my Air pretty empty--I don't have any music or photos on there, just all my work documents--while the MBP is pretty much a mirror image of my iMac.
I try to give all three machines equal attention.
you can't use the Air for everything.
Like recording TV for iPhone conversion...
or to house your bazillion gigabyte library of music, movies and TV...
I have this setup except without the MBP... no need really since the combo of the iPhone and MBA handle most all of on the go tasks. The Air is easier to lug around to starbucks... one bag of work versus 2.
Im on the fence about getting one... If I got one, it would be one of those refurb $999 ones. I keep hearing craptastic things about it, though, so Im not sure what to think.
My advise is to spend a little more, and buy the new one; sometimes can be found on eBay for only $300 to $400 more than refurbished. It will definitely be a much better value in the long run. The new MacBook Air is my primary computer. The old MBA was NOT capable of being my primary computer. Many have different experiences, but I would advise you to read some other threads to determine which one is right for you.