I went for a 5 mile run in the new Nike Flyknit Trainer yesterday.
It is built on a much narrower last than Pegasus, Lunar line etc; closer to Streaks.
Good cushioning in the right places, very comfortable upper. I went for half size below normal Nike sizing based on trying the Flyknit Racer and reading that both models are a bit big.
Overall I am very impressed with the shoe, supportative, cushioning but not mushy. At 7.5 ounces (I think) it's between a racing flat and regular trainer. I'm not sure how the durability will be obvioulsy.
Since I do like it I will try to stock up as Nike will surely change it drastically! Once a few fatties buy it, complain it is too narrow, they will change to accomadate them.
I was lucky to get the white with black special edition, although I don't really care too much about color.
I would say it would be good for regular runs and faster paced runs. Maybe not that great for trails or grass based on the sole. Likely a good treadmill shoe as well.