By using Metamod I didn't have to compromise the website with the lowest common denominator browser in mind (eg for me, ie6). metamod detects which browser is being used and displays your specified choice of modules for that browser. Eg. The user has ff3 or ie 7, (s)he sees that sexy new module, if they have ie6 the site still works but you automatically substitute sexy module for sturdy tried and tested module.
Everybodys happy and you're not tearing your hair out because you've found out at the last minute that your site doesn't work with ie6 and 23% of your visitors are still using it!