I think its somewhat accurate. I've settled into being ok with Impaler's approach. I don't use the Workout feature, and instead let the Activity feature track Move, Exercise, and Stand on its own. For gym based activities on an elliptical trainer or treadmill, it's generally been accurate but not always.
I know what I've actually done for exercise outside of the gym (use a dedicated activity/sports tracker app for rides, hikes, walks), and from what I've heard the health/fitness tracking capabilities will be more accurate with watchOS 2.0.