I'm not into making recommendations, but I use it -- 750mg XR (extended release) for longevity, for now -- I'm likely to switch to Rapamycin later this spring at which point I'll probably drop the Metformin. I'm 60yo, lean and active (lift weights and am training for a marathon), and only "pre-diabetic" to the extent required for a doc to sign off on the prescription.
I take Metformin along with TRT and Semorelin Glycine. I had no problems with nausea, diarrhea, or any other side effects when I started it, nor do I now, although I understand that some are not uncommon, initially. Despite what some claim about it interfering with muscular hypertrophy, I am adding muscle mass, have no problems in the gym or doing cardio.
FWIW, my cortisol levels are up a couple of points to 8.6, since beginning Metformin. What or whether there's a causal relationship there, I cannot say. My IGF-1 is up from 102 to 149, although I think that's more related to the Semorelin Glycine.