Is it a good thing when a guy says he feels like he has always known you?

It is a line. There is no way to know EVERYTHING about someone, but you can be comfortable to be with. Keep listening to what he says and make your own opinions. It's possible it's the truth, but a lot of people say that when they are really attracted to you. It goes away when he or she and yourself find out that they know little or nothing about you. Most of the time, it was just infactuation, and the feeling goes away at your first disagreement. It's a great line though!