You'll probably notice some lag, but does it matter? It seems like your mind is made up to buy an underpowered machine.
Can you ever offer anything constructive without trolling? Its only "underpowered" to someone who knows nothing about computers.
To the OP, as someone who OWNS this computer I can tell you a few things:
First, I am a power user. The Mac Pro in my sig is my main machine and I use the following daily:
-Maya
-XCode
-Unity
-Logic
-Handbrake
-Houdini (sometimes not daily)
-ZBrush
-Corel Painter
-Photoshop
-Aperture (sometimes not daily, depends if I just had a shoot or something)
-BOINC
-iTunes
-VMWare
-Realflow (sometimes not daily, only when I need to simulate liquid or something)
-Adium
Usually these are all open at the same time and a lot of programs like Handbrake or Maya are batch processing. I'm not some person who thinks just cause they use Photoshop that means they are a power user.
That being said I use my Macbook Air as a supplementary machine. I see a lot of misinformation on this thread by people who don't know much about computers. The people haggling over specs are WRONG. The SSD alone offers HUGE performance increases in this machine. Its not going to be the fastest for someone who wants to do video encoding but I can comfortably run Maya, ZBrush, Houdini, Unity, Painter, Photoshop, XCode, and Logic on the machine. I know because I've done it. Sometimes when my main machine has a ton going on it I hop to my air to work on some more things.
The Macbook Air is fast, light, powerful computer and can be easily used as a primary machine. I use it all the time as a primary machine when I have to travel for weeks. No regrets at all.
Don't listen to these people on this thread who obviously don't own the machine or have never put it through its paces.