Impact factors, what are they good for?
The following shows the distribution of impact factors (IF) of journals in the categories 'mathematics' and 'mathematics, applied', for the recently released impact factors calculated by Thompson Reuters. They write
This is a box plot of the subject category or categories to which the journal has been assigned. It provides information about the distribution of journals based on Impact Factor values. It shows median, 25th and 75th percentiles, and the extreme values of the distribution.
That outlier at about 7.2 is a new journal in numerical methods that hasn't published a lot. The other, much smaller points above the 75th quartile are journals like the Annals of Mathematics, Journal of the American Mathemtical Society, Communications in Pure and Applied Mathematics, Acta Mathematica -- which, I should point out, have a similar effect in one's CV that a Science paper does for other areas.
To compare, the mean IF for the subject area 'multidisciplinary science', which includes glossy mags such as Nature and Science is about that of Annals of Mathematics. I would posit it is much, much harder to get a paper into the Annals than a mid-ranked-by-IF general science journal.