How accurate is it?

This is NOT "study-level" ATC. While it's awfully good, you will occasionally experience some weirdness (bad taxi instructions, for example), or it may ask you to do something that's not 100% correct according to standard guidelines.  We watch the logs constantly and are regularly improving the system. We are committed to making the experience as realistic as possible, but we also know that we will never achieve full "study level".