No direct personal experience. It's not something I've ever seen listed as a side-effect. I checked my copy of Bilous and Donnelly and it's not included there.
However this research was published in the BMJ in 2010 - it found some impact on B12 and (at first) folic acid. However once they controlled for smoking and BMI, the impact on folic acid changed.
Objectives To study the effects of metformin on the incidence of vitamin B-12 deficiency (<150 pmol/l), low concentrations of vitamin B-12 (150-220 pmol/l), and folate and homocysteine concentrations in patients with type 2 diabetes receiving treatment with insulin. Design Multicentre...
www.bmj.com
Compared with placebo, metformin treatment was associated with a mean decrease in vitamin B-12 concentration of −19% (95% confidence interval −24% to −14%; P<0.001) and in folate concentration of −5% (95% CI −10% to −0.4%; P=0.033), and an increase in homocysteine concentration of 5% (95% CI −1% to 11%; P=0.091). After adjustment for body mass index and smoking, no significant effect of metformin on folate concentrations was found.