When I'm at my desk, the laptop is closed and connected to a beast of a monitor, external keyboard and mouse. It becomes a workstation. But when I want the mobility, it's there. Even if only to scurry off to a unoccupied conference room for bit of silence, it's very valuable for me. It's easily worth the ~700$ above the cost of a immobile box every two years.
As for performance, I've yet to feel constrained by the Air. But then again, I code mainly web apps in ruby, so no fiddling of thumbs while things compile - the most of time is lost waiting on database server. And my brain to come up with a solution.
I really hope to attain that level of productivity and mobility, and I understand that some output is better than no output--When I am working I want to be using the best tools I can reasonably afford, and that for me is a desktop with a few monitors and plenty of space to spread things out.
Now when they release a high-res macbook pro I might get one, but for now I am sticking to a desktop.
To me the ability to pull up my laptop and go work somewhere else for a few hours, or have my main workstation with me at conferences, etc is well worth sacrificing some power. Sure I can make use of multiple large displays, but I can do a lot of work without them without losing much efficiency. For stuff like design where I really need it, I can wait til I get home.
Granted, I only ever used my Linux laptop for programming and light web usage. I never tried to set it up for anything else, but it was perfect for that one purpose.
Sony Vaio F12: quad core, 8GB, SSD, 23" LG travel monitor (5.5 lbs, nice!), iRizer laptop stand. Cost: 1/2 MBP. Code warrior: I be
The only thing I miss from OSX is Finder; well, the window manager as well; let's not forget the spinning beach ball either ;-) People, Linux is a gift from above. OSX is great for consumers, and is de facto the best OS on the planet, but Linux is the best OS in the universe -- big difference...
Now, I'm a sessile work-at-home startup guy who's really happy with a Linux tower and several displays connected. I've gone so far as to miss the ubiquitous color beige of ages past.