The major problems with Metformin are that it frequently makes people nauseous, it causes sperm damage in men (effects go away when you cease taking it + about 3 months), and in rare cases can cause lactic acidosis if you consume alcohol.

Admittedly, I got put on it because my blood work wasn't going in the right direction, but it has noticably increased my ability to lose weight. Not exactly scientific, but in my first 5 months of trying to lose, I was down 30 pounds, in the next 5 months after starting it, I lost about 50 and was less strict about my diet...

Now I'm down a full 100.