especially if you mean by "usability" that it actually works
Review: Ubuntu's New 'Gutsy Gibbon' Brings Linux Out of the Jungle
The familiar old script that Linux is only for geeks has been largely rewritten recently with the arrival of Ubuntu, a version of Linux for the average user. In its three years on the scene, Ubuntu has quickly gained a reputation for being easy to configure and use.On Thursday, Canonical, the London-based company which acts as Ubuntu's commercial sponsor, released version 7.10 of the software. This latest release, dubbed "Gutsy Gibbon," proves that Ubuntu Linux can compete with and, in some cases, trump Windows as an everyday desktop system when it comes to pure usability.
p.s. -- It is, of course, absolutely free.