As of yet I'm not aware of any third-party software that will fully utilize XMP metadata. A shame since it's an open standard (XML) but understandable since XMP itself is still rather new.
This is certainly true. Until Bridge's advent, I used Extensis Portfolio mainly because of its extensive keywording and custom-fields capabilities, with accompanying search features. (My indexing scheme is keyword-dependent.) But Extensis has really dropped the ball with its .xmp metadata handling interoperability with Bridge, such that xmp metadata written to sidecars in Bridge does not reliably show up in Portfolio. So I've all but abandoned it until Extensis gets its act together.