I saw a study a few months ago on that, I think it was linked on Overcoming Bias,but I'm not sure. Apparently, most people rate themselves in the 3rd quartile; everyone under average thinks they are somewhat above, and people in the upper quartile generally over-estimate the average, thinking most are more like themselves, so only those who are actually in the third percentile are (accidentally) accurate in their self-assessments.

It's a lot like everything. I haven't found any skill or talent where that isn't true.

Keeping this in mind is the first step towards a reasonable self-assessment.