I just don't like the way it was implemented. I always found myself hitting the "adjust vc" or whatever you want to call it button before I made every throw. I really didn't notice a difference between qb's because it pretty much added an extra button press for every qb.
I would rather see some other sort of system. Maybe having receiver icons pop up at different intervals, simulating a qb going through a progression. The icons could come up in a faster rate for better qb's and a slower one for players not known for their decision making.
I really don't the right answer and I think the cone could work; its just a hard thing about football to try to simulate, especially since most players have a very limited understanding of what real "reads" are and what the difference between being open and covered is in the NFL.