I played with both the 11" and the 13" MBAs in the Apple store today, and I hafta say I was very impressed, especially as I feel the prior MBA was DOA, and I think sales numbers confirmed that, but this is a different beast altogether.
I use both platforms, Windows and OSX. My corporate machine is an aging Dell M90. My home workstation is a 3.06 27" iMac.
I'd looked at the iPad earlier in the spring, but passed due to many of the reasons normally cited. To me, the new MBA is the perfect machine for people that are intrigued by the iPad, but really need a full-up computer.
I woulda bought one today, but they didn't have any 13's in stock with 4GBs of RAM. They'll get them in within a couple days.
The weight isn't that big of an issue between the 11 and the 13. The screen is so much more useful on the 13 as is the built-in SD slot, perfect for my photography without needing to carry an adapter. Oh yeah, and the screen of the 13.3" is a 16:10 aspect ratio, so a little more vertical space for open windows. Apps under Lion running full-screen are going to be sweet...
I think the 4 GB will be needed for Lion and as the RAM isn't user-swappable, I don't want to hamstring myself now and regret it later when it's only $100 for the extra 2 GBs.
As it is, the 11" with a 128 GB SSD upgrade and the extra 2 GB of RAM close enough in price to the base model 13" with RAM upgrade to make it a no-brainer , for me at least. To each his own.
As others have said, I don't think the numbers for the processors tell the whole story and the instant-on makes the whole package seem screaming fast. I'm not keen on the 256 GB upgraded SSD, as the price is steep and the difference can buy a couple nice external HDDs.
I think Apple hit one out of the park with this one. Not a fan boy, just want to get work done and I think this rev is cherry.
Tony