Differences between "IS NULL" and "ISNULL()" in Mysql

This thread is similar, though not exactly on MySQL. According to the test shown there:

IS NULL is more efficient as it doesn't require a scan.

Seek is generally faster than a scan as it only includes qualifying records, while scan includes every row. It is explained in more detail here.

Another difference (though it's not performance) is their negation syntax:

IS NOT NULL  /* using NOT operator */
! ISNULL()  /* using exclamation mark */