Weirdest 'side effect' I got from Metformin was, I think, actually the result of my body fixing that whole IR issue...food started tasting completely different!
Stuff I'd hated all my life suddenly tasted great. For all my adult life I thought raw tomato was bitter and disgusting; suddenly I discovered it was yummy, and my fridge almost always has tomato in it now - I love fresh tomato on toast with basil leaves.
Same thing happened with spinach, same with...oh, lots of stuff. I even discovered I quite like carrots in small doses, although I'm still disinclined to munch on carrot sticks. Weirdly, carrots taste sweetish to me now, where they were always very bitter.
It was the most peculiar thing. It was like once the insane craving for sugars went away, my taste buds just opened up and discovered 'new and exciting' flavours.
It's not noted as a Metformin side-effect, so I really can't provide any good medical reason why it happened, I'm just glad it did. Made eating better a heck of a lot easier, I can tell you.