It looks really promising. However, there are some growing concerns I believe that link it with mitochondrial dysfunction as well, if I recall correctly. Hopefully that's not the case, but it's a developing story. (It does reduce exercise output as well if someone is an athlete and cares about hat, but since it does mimic exercise in a way on a cellular level, that's to be expected I guess). :'(
There's always exceptions in popular medicine. No doubt a small minority of people get no benefit, or may even be mildly harmed by the medicine. But Metformin, in particular, is one of the most popular drugs in the world, and it's strong safety profile is well established. "All-cause mortality is lower in people on Metformin". So, I don't think there will be any big surprises found in its safety, but maybe we'll find that a certain minority of the population aren't suited for it.
Mitochondria need proper care too. I take iodine and magnesium for asthma (which is cured) and I hear iodine is also go for mitochondria, which might explain why it's good for so many things.
