Interesting that I seem to be the only 17-inch fan here. I can see the attraction of an MBA, but I prefer having a screen big enough that I can do everything on it without an external monitor, and I don't mind lugging it around at all.
I'm 3.5 years into owning a 17" MB Pro, and my next laptop will be the 13" Air. I wanted the screen real estate when I bought it (coming from a Toshiba that could only do 1024x768). While I love the performance and all the pixels, it's just not portable. The higher resolution on the Air is good enough, and the weight is very appealing.
I travel between three offices every day on foot / bike. I tried the 13" MBP on my back for a while, but in the end it caused more pain than it was worth. I can only imagine the 17" being far worse.
For now I use a Lenovo x200, but it's about the same as the MBA 13". So while sure I could "lug" the 17" around, I'd much rather loose some screen space and save my back. But I will say, I am quite jealous when traveling of the screen space the 17" provides.
The difference in weight is much more than it sounds.
Work had issued me a Lenovo Thinkpad T61p with a large extended battery pack to make it last closer to 4 hours. It weighed about 7.5 pounds. After just 15 minutes, my shoulder would become sore from lugging my bag around.
I handed them their laptop back and bought a 15" MacBook Pro with my own money. Just that 2 pound difference means that instead of my shoulder being sore after 15 minutes, it takes about an hour for it to become sore.
If I didn't need the memory and the CPU (I do data warehousing work, and need to run VMs almost all the time) I would certainly have gotten a 13" MacBook Air to shave off another 2.5 pounds. Heck, I'd probably get an even smaller bag just to shave off another 1/2 a pound. It's absolutely worth doing.
There's a difference between "I dont mind lugging it around" and "I lug it around a lot". I had a 17 inch once and realized that, even though I could move it, the weight and size made it inconvenient. It ended up sitting on the desk all day.
For me it's less the lugging (though that's a factor), but more the easier ability to use the machine in cramped quarters, like at some coffee shops, and in economy-class plane seating.
17"MBP here too, and after 2 years its been a great machine, still decent specs by today's standards. But the size is starting to wear on me. I've entertained the option of upgrading to a new 15"MBP but it doesn't seem like a big enough jump to justify the cost.
I hope Apple has a 15" Air in the works because that would feel like an upgrade in multiple dimensions - cpu, ram, display, weight, thickness, portability.
