Metformin has been used for quite a while to help with insulin resistance. It has some metabolic effects that other oral drugs for diabetes do not share. Typically it doesn't cause severe low blood sugars for example. (unless you are having a low anyway).

It is supposed to help with blocking some gluconeogenesis by the liver which happens during fasting states. And to enable insulin to get glucose into cells more normally.

If you do get LOWs pay attention to them.

The people who get the diarrhea have slightly more serotonin in the GI tract than others. Metformin increases serotonergic responses and motility of the GI tract.
The side effect may come after more than 3 months on the drug. Also using a timed release may help some. I've seen people take 2000mg of immediate release daily with no problem.
But if you are one of the sensitives (and there are many of those) you will suffer. And it may begin as low as 500mg ER daily. This effect does NOT go away... so don't let your doctor try to get you to keep at it. It will in fact get worse and worse. It will interrupt sleep and put a huge crimp on leaving the house. (even when using Imodium).

When it works, it is great. But the "other"? It can be very agonizing.