I take it on about half my weekdays at 5 mg and it really helps with my workouts. I'm able to push longer on runs, go faster, and when I do weights I feel way stronger.
Made my heart race, even sleeping. Never hurt though. Discomfort is tied to how aware you are of your body. If tuned in or anxious you might read too much into it. I just tracked it. Got ECG and Holter to verify no damage. My heart might have more glucagon receptors or they're more responsive.