What sets netbooks apart from laptops is that you sacrifice speed and screen size for battery life and portability. "Cheap" (as in shitty construction from shitty parts) is found in all form factors, and you tend to get what you pay for. For the same build quality, you'll tend to get netbooks cheaper than laptops primarily because the netbook requires fewer components, uses a slower processor, and has a smaller screen. But you can still find good quality netbooks just as easily as you can find crappy laptops, and vice versa.
"Netbook" may be a bullshit term, but it's here, and it accurately describes a niche of products. They used to be called "ultraportables" or "subnotebooks" when they first came out in the 90's, and there's still so much overlap between the "netbook" niche and the "subnotebook" niche to make any distinction pretty much meaningless. The basic theory is that a netbook will tend to sacrifice even more power than a subnotebook because it's designed to connect to servers to do most of the heavy lifting, and it really shouldn't have a lot of storage, but most have plenty of power to run local apps and tons of storage.
The build quality of my wife's Asus eee is quite good. It's plastic, of course, but it feels like durable stuff. It gets around 8 hours of real-life-real-use battery, it weighs almost nothing, and there are very few things it does noticeably slower than our dual-core 3GHZ desktop beast.
Yes, they've moved from a nice low-power SSD back to a spinning drive to save a few bucks, which was a bit disappointing, but it's still a solid little bugger, doesn't generate a lot of heat, and it still lasts all day with no need for a recharge. It's no replacement for a desktop or even a laptop, but it's not meant to be. It fills that niche between "portable as a cell phone" and "powerful as a laptop" quite nicely, and for a lot of people it's all the computer they really need on a day-in-day-out basis, and can be carried around a hell of a lot easier, and you're not constantly looking for a power plug when you do carry it.
I didn't buy it because it was cheaper than a laptop, I bought it because it is more portable and holds a charge longer, and because any laptop on the market today is to big, too clumsy, runs out of power too fast, heats up too much, and has more power than you need for what my wife wanted it for - casual surfing, maintaining a few HTML websites, and keeping up with Facebook.
"Too much of a good thing is not a good thing".