When I take my BGs and if I get what I consider to be an implausible result (granted, not that often), I just take it again from another finger. That nearly always results in a more appropriate result.
I was on Gliclazide until about a month ago and only ever had a 1 -2 mmol/l improvement over whatever other meds I was taking and when I came off it, only had a very modest deterioration too. So, for me, it didn't make anything like that much difference - but I titrated the dose up gradually too, so my improvements were at a very modest resulting pace too.