I did persevere for 11 weeks as I was told it has lots of benefits. I had headaches, nausea and frantic runs to the loo. It never showed any real plus for me so I don't take it now.
There is a slow release version which may be kinder to you.. more expensive, hence the reason for GP's not prescribing it first.
I wouldn't like to ignore migraines.. if you think the Metformin is causing them please tell your GP