I like flies... if they are done correctly and tension is kept on the chest (not going all the way up and resting). Also, I agree that a person shouldn't make it a habit to go super low on flies. 90 deg, or just a tad lower should be plenty.
Agreed. Flyes used in conjunction with compound pressing movements for chest are, in my opinion, the best way to train the chest. I tend to save my flyes for the end of my chest routine, usually done in a superset with something like cable crossovers.
But like Shakes said, it's all about tension (contraction). You want to keep your chest tense, or contracted, throughout the entire movement. I do like to get a good stretch at the bottom of the lift, but there's a differene between a good stretch and stretching too far. You kind of have to figure out your range, but a tad lower than 90 degrees is about right. Also, at the top of the movement, I like to squeeze my chest muscles for a brief moment, causing further contraction and muscle fiber stimulation.
Try doing a superset of incline DB flyes followed by cable crossovers, imploying the techniques described above, then after your superset, flex your chest as hard as possible for about 15-30 seconds. I love it!
The chest is a tricky muscle group to train. Some people are gifted enough to where they only need a handful of pressing movements to develop a full, powerful chest, while others, like myself, need a multitude of exercises to hit the chest from as many angles as possible. Trail and error until you find your groove, then consistency. That's my two cents anyway.
Happy training!