I've gone through 5 iPad Air 2, and I decided to settle with my current iPad Air 2 Gold 64 GB Cellular (the 5th one). This iPad is almost perfect: It has blue tinted screen (cool color tone) instead of pinkish tinted as others. I extremely hate pinkish tinted (warm color tone) screen. This was one of the reasons I exchanged iPad 3rd 13 times in 2012. iPads 3rd were pinkish, yellowish, dead pixel, light blessing, etc.
My 1st iPad Air 2 Gold 64 GB Wifi Only, which was the one I was eager to get at my nearest apple store 2 days before the store officially sell iPad Air) was warm color tone, and it has shadow along the left side (called book spine).
My 2nd iPad Air 2, also Gold 64 GB Wifi Only,was the one I pre-ordered on apple.com on 10/17. Its screen was almost perfect: uniform cool white, nonexistent book spine. I should have kept this iPad if I had not changed my mind paying the difference ($130) to get a cellular version.
My 3rd iPad Air 2, or rather my 1st iPad Air 2 Gold 64 GB Cellular, was quite bright, but it was also too warm. It looked pinkish.
I stayed with the 4th one only 15 minutes, and I immediately came back the apple store to return it. After I came home, I purchased the 5th on apple.com on Wednesday 11/05 and I received it on Friday 11/07. It looks perfect with blue tinted screen.
Finally, the 2nd and the 5th are perfect because both of them are "DM" while others are "DL". My nearest apple store and bestbuy have only DL, so their prototypes look quite pinkish. I guess there are various display suppliers (LG,Samsung, Toshiba, Sharp..), and DM and DL use completely different supplies.
If you want to buy an iPad Air 2, get a DM one