I have been taking both metformin and avandia combined for quite some time now. Metformin is the generic form of glocophage (sp) which gave me terrible diarrhea, and eventually the glucophage was not digesting in my stomach so it was not working. That's when the doctor but me on Metformin. It works much better... but your stomach might start grumbling and growling at you from time to time, along with a little diarrhea. But don't worry; this is few and far between.
The Avandia... well, I don't think I have any side effects of any kind. But I read on another website that it causes liver failure and heart failure... whether this has been proven or not is beyond me. So, be sure your doctor continues to do regular blood tests to check these areas.
I think both are very good and seem to control sugar very well.
Good luck.