This used to be simpler, but not very powerful. Now it's really powerful and useful, but it may not be obvious how to use it...
To search for bugs associated with an email address:
You can look for up to two different email addresses; if you specify both, then only bugs which match both will show up. This is useful to find bugs that were, for example, created by Ralph and assigned to Fred.
You can also use the drop down menus to specify whether you want to match addresses by doing a substring match, by using regular expressions, or by exactly matching a fully specified email address.