I had a 1st gen Macbook Air.
It was a lemon. Brought it back 2x for various issues - such as CPU pegging, overheating, etc.
And when returned from the Apple store, the shell was mating properly.
After the second return, I stuck it out as I felt there was a refresh coming and that refresh would overlap with my Applecare. It was painful though to use the MBA that summer since I had to watch the laptop temperature.
Sure enough after the MBA refresh, when I brought it back to the Apple store...instead of repairing it for the 3rd time, they just gave me the refresh model.