I've been running in different lightweight Nikes for years. I wasn't sure about the no-lace Flyknit at first; thought they would loosen over time (but I found them for a really good price). I have been pleasantly surprised how well these shoes are holding up. They are still as comfortable as the first day, I currently have 227 miles on them.
I really only use them for track and treadmill running, which they are perfect for. They are lightweight, but they are also thin. You will feel every rock on the road. I typically run about 2-4 mile workouts with these shoes.