When it comes to X11 and OSX, the word 'seamless' isn't one I'd use. Although you can use the same fonts most of the time, other than this there isn't much integration at all. In particular, X11 doesn't have a standard set of GUI widgets, so every app either rolls its own or (more commonly) uses one of a number of toolkits, none of which are very Mac-like. This said, depending on what you do, there are some good X11 apps out there. The most famous is probably the GIMP, which is argubly the most powerful free image-editing app out there on any platform. Also popular is NEdit, a text editor along the lines of TextWrangler or BBEdit but with many more plugins and syntax-highlighting modules.
There are many other X11 apps out there, of course, but most are either highly specialized (such as the GRASS GIS system) or already have ports and/or competitors on OSX (such as X-Chat, an IRC client). GIMP, however, is popular enough that many people install X11 just to be able to run it. I know I do.