X505
I have an X505 and have looked at a MBA in the Apple Store on Regent St, but didn't find enough there to buy one yet. I really really like the X505 - I use it every day as it's so portable that I can chuck it in a briefcase, bag or rucksack and not notice it's there. THAT is portable, to me.
If you're getting a new laptop now, you can pick up the X505 for under £500 on ebay and a MBA for over £1000 - that's quite a price difference IMO. The MBA is faster, has more memory, more screen real-estate and more hard drive space so that is what you pay for and I'd say it's worth it. If you don't need anything particularly fast then get the x505 or an Eee but if you need a bit more grunt, fork out for a MBA.
I've not played with an Eee, but apparently the keyboard leaves something to be desired - I don't particularly like the MBA keyboard but it's passable. I really like the X505 keyboard, over normal keyboards.
In other news, I hardly ever use anything with my X505 (network cables, headphones, external monitors, CD drive, DVDs) - almost everything can be done using wireless internet or a USB thumbdrive and the MBA can cope with those.
My ideal is a laptop under 1kg and about as thin as the X505 or MBA, so the MBA is a bit of a porker in my book!