The free version has the ad in the upper right and is not that annoying, but I finally bit the bullet and bought it. The license is good for Windows and Linux and on every computer you use. It has the tabbed browsing that Firefox users like, as well as "mouse gestures" that let you navigate almost intuitively.
One feature I appreciate is the magnified screen effect that's easier to accomplish than on IE.
Two features Opera has I wish Firefox would adopt are the ability to move the tab arrangement, and to easily resize each tab.