The screen size is okay for me. And that's coming from 1920x1200 on my 17" MBP. I've had much less trouble coding (using Eclipse) than I thought, due to the smaller size.
I LOVE the keyboard. I think it has a better feel than my MBP. The only "issue" is that you have to be very careful to not put pressure on the kb with your right thumb when typing, or you'll "click" the touchpad and make the cursor jump and/or lose focus. I've trained my hand to not do that anymore so it's rarely an issue.
As for space, I have 128 gig and it's tight. 256 would let me keep all the files with me that I want. For now I have a 1TB external for that, but it's not convenient to use unless I'm at a desk and plugged into power since it uses computer power, and would suck the battery down even more. Likewise, on battery, I don't often use my Verizon USB 3G modem; instead I turn on the WiFi hotspot on my Droid X phone, since it preserves the MBA's battery.
Memory-wise, with 4 gig, it also does surprisingly well. Right now as I type this, I have seven copies of Chrome running, one copy of Firefox, one Terminal, Eclipse, the new Mac App Store, Forklift, VirtualBox with a running Windows XP guest session (which I gave 768 meg of RAM), which is running MSSQL. I also have ColdFusion 9 server running, Skype, TweekDeck, and TextEdit. I'm only 225 meg into swap, I have 1 gig inactive, 2 gig active, and 1.07 gig wired (whatever that means).
Rob