I tried metformin for a couple of months years ago and a very small dose (125 mg working up to 500mg a day) made me very ill. Nothing specific, just bone tired, weak, dizzy and "unwell" or lethargic. I didn't seem to get the GI side effects, but that was probably because the dose was so low and/or I already ate a very healthy diet.
I put on 5 kgs in the two months whilst I was on metformin I also lost the ability to control my weight. I had been a normal weight prior to starting metformin and afterwards I would every now and then quickly put on 5 or 10 kg within a very short space of time (generally a couple of weeks or a month), without any apparent reason (no change to diet/exercise/activity levels). My irregular 6-8 periods a year instantly turned into none for 3.5 years as well.
I managed to get everything back under control when I found out about d chiro inositol and started taking it. My periods returned a month after I started taking it and after 3 months I started losing weight, very slowly. I still have a way to go before I'm back at my "pre-metformin weight", but at least it's heading in the right direction now.
Definitely be careful. Metformin works very well for some people (generally those with a BMI of over 35 according to current clinical guidelines) but for some people it can cause a LOT of problems.