I've been taking Metformin ER for over a year now, and I dropped 40 pounds over 10 months or so without any real changes in my diet or exercising. Unfortunately, I started binge eating and gained it back.
Now that I've stopped bingeing, I believe I will lose that weight again. I will say that it is difficult to take due to the nausea it causes. This only seems to be a problem if I don't eat enough with it. The sweet spot for me is about 400-500 calories, so I take it at breakfast and dinner. I also had to increase my dosage 250mg at a time, over the course of a month, because starting at 1000mg made me feel like I was going to die! It's a great medicine though, once you get past the side effects.