It's thinner than I'd ever had thought, despite the pictures - and lighter. People say it doesn't make a difference when you're carrying it around all the time. This thing is lighter than my trainers!
I've got the SSD version btw... which boots from power on to OS X (or XP bootcamp for that matter) in seconds.
Someone in work has an MBP and I had an MB. It is quite nippy, but it's definately (and noticeably if you do a lot of heavy stuff) not as powerful as the MBP, but comparable to the MB. I've got parallels running with 1Gb of the ram and it all works fine. But it isn't lighting like the MBP.
At the end of the day, it is a perfect travellers machine. I simply love the fact everything is external... external dvdrw, external ethernet etc. It means I rarely have to carry anything I don't need.
If you want power, look elsewhere. The MBA isn't slow by any means, but it isn't fast!
Any specific questions I can try and answer them. It does get a tad warm, too, but so did my MB.
EDIT - also, with wifi on/bluetooth off and brightness on 60% I got under 4hrs battery... slightly disapponting.