I've tried metformin last year when i was still natty.
I started with 500 mg once a day at dinner, then gradually increased to 2000 mg/day on my high carbs day only, usually splitted into two doses of 1000 mg.
Had no major sides, probably a bit of stomach discomfort at the beginning.
I've found that it kept my muscles fuller on very low carb days and on carbs refeed it prevented spillover very well, shuttling all carbs inside muscles. Much less fat gain on prolonged high carbs diet too.
The day after a carbs refeed plus 2000 mg of metformin the pumps in the gym were insane and even painful, muscles were full, hard and vascular.
No lethargy, rather i had an energies and endurance increase.
I stopped taking metformin since i've read that it can mess up with androgen receptors signalling and it leads to an SHBG increase, reducing free test levels.