The negative from this is that it stores glucose and water as glucogen in the muscles for future use as a local fast access energy store. If we are sedentary and do not use this up, then it tends to stay stored, along with the associated water in the cells, leading to weight gain. Not such a problem as the other storage cells in the adipose cells and liver, but this weight is the kind that WW and other sliming diets are geared to reduce quickly so giving impressive success stories. This is why weight lost by diet gets regained quickly again since it is just sugar and water as glucogen otherwise wrongly referred to as fat.
Insulin also opens the door for lipid fats to be stored in the liver and adipose tissue which is how metabolic syndeome starts - this fat is true fat and much harder to shift. Glucose can also become converted into s form that can also be stored in the adipose tissue but that is a slow inefficient process that only occurs when sugar levels are low(ish) as it is a starvation protection mechanism. These last two are not really affected by Metformin being there or not, so it is muscle IR that it can help to reduce. If bgl numbers are the thing that rows your boat, then Met is good. If you look at T2D in the round then it may help a little, but not a lot since what goes in tends to stay in if not used. Of course the other route is for excess glucose to be filtered out and excreted in the time honoured tradition for T2D - peeing a lot, This obviously takes time and is not a quick fix either. Some T1D get put on Met too but that is for overall protection rather than fighting bgl levels and IR.
Metformin has other interesting properties and is apparently helpful in providing protection to the cardio vascular system as a whole.
