Agree with you about the construction. A little over two years ago, I bought a Sony Vaio laptop for about $800. It was powerful enough, but I used it for about 5 months and had to go out and buy something else. It was heavy, the hinges started getting loose and creaking/showing wear. You had to really bang on the keyboard while typing, the entire thing was HEAVY, the trackpad would randomly freak out, and it was basically unusable without a mouse.
I went out and bought a $900 11" Macbook Air a few months later, and it was an absolute pleasure to use in comparison.
Now, I rarely use the Sony laptop -- the keys have started falling off (I gave up reattaching the 'z' key), the screen is "floppy," and I dread trying to right-click on things with the trackpad, but it's now a dedicated dev environment for my job, on days that I work from home. In comparison, my Macbook Air has gotten far more abuse. I toss it around, carry it in my purse, I've lost it under couch cushions -- it has a dent on the bottom from who-knows-where, but it still feels as nice to use as it did on Day 1.
The only time I've run into performance problems is when I'm running Eclipse + Tomcat + heavy MySQL load + some random Python scripts chugging away + ten billion tabs in Chrome. The baseline model 11" air is a little sluggish then :-p But really, I'd rather upgrade to a more expensive model, or pause execution on some things, or clean up my processes, and/or just offload processing to an EC2 instance than ever have to buy another crappy-feeling plastic laptop. It just makes an unbelievable amount of difference in day-to-day use that I don't think people give enough consideration to.