My point wasn't to give the overall pharmacology of metformin but rather to inform MAX that combining the 2 is common practice and that metformin typically decreases the amount of insulin needed. Aside from your personal experience and opinion of Metformin, the MAIN mechanism of action is decreased glucose production from the liver (decreased intestinal absorption is a minor effect although maybe for you personally it may have a more pronounced effect). The biggest benefit of this drug is that it does not cause direct hypoglycemia like the sulfonylureas (glipizide, glyburide). That is of benefit to athletes. The actual bigger concern with Metformin is it's propensity to cause lactic acidosis which can be fatal. One way to reduce this risk is to stay well hydrated. During truly intense exercise, i would consider skipping my dose beforehand.
All in all it is difficult to manage diabetes as it is and intense or endurance activity makes it more difficult to manage.