Just to be clear: I absolutely believe that Metformin can be very good for your overall health in many ways. Period.
Now, the use in Bodybuilding.
Metformin in a dose-dependent manner decreases glucose transport from the intestinal lumen into the blood. Considerably. You eat 400 grams of carbs, but that's FAR from what you're absorbing. I'd just eat less carbs and skip the damn GI distress that comes with said malabsorption.
Also, what concerns me big time is that at least a few studies demonstrate that insulin-stimulated glucose uptake is not augmented, but rather tends to be DIMINISHED in skeletal muscle from type 2 diabetic patients following metformin treatment . May or may not be extremely relevant for healthy active non-diabetic people, but still...
The glucose clearance can increase by up to ~75% (!!) in the intestine of acutely metformin-treated animals. This signifies that the intestine acts basically as a glucose sink following metformin treatment. Is this what we actually want?
Last, my own personal issue with it. No matter the dose or how slow my ramp-up dosing schedule is, I always get very loose stools that don't subside even after a whole month or more. Taken with or without food, and as low as 250mg/day.
Well there's no way I can look at that mess and think "it's all good, it's not severely impairing my other nutrients uptake".