The combination is possible, but some additional effort is necessary. Ask google for experiences... You should only choose this solution, if there is a real reason for it.
JSF has also many "fancy components", look at component libraries such as RichFaces, MyFaces, ICEFaces and so on! Only the implementation of the standard itself does not contain any "fancy components" :-)
By the way: GWT itself also contains only simple components. If you want to use some nice components, you have to use component libraries such as ExtGWT or SmartGWT.