Like everyone else said, Hardware numbers are very accurate when compared to the figures Nintendo/Sony/Microsoft releases. Software, like TheSource said is harder to track but they are still usually within 25% in the worst case (excluding outliers, excluding incompletes which are usually older games).
@DirtyP2002 - unlike the NPD, VGChartz adjusts their numbers when they get better data. It doesn't prove much to compare month to month VGChartz data to NPD data as a few weeks later numbers may be adjusted. While NPD gets way more data than VGChartz they still use heavy extrapolation to arrive at their estimates. The only real way to gauge the accuracy of this site is to match it up to official shipped figures, which like has been proven match up quite well.
