I have used Extensis Portfolio for nearly 2 years. It is powerful and flexible. It has a lot of display options, and its search capability is very good. My classification scheme is heavily dependent on keywording, which Portfolio does well. I like its interface better than Bridge. I like a lot of things about Portfolio.
My major complaint about Portfolio--and it's a big one--is that it doesn't seem to handle .xmp metadata very well, while Bridge does this smoothly. Because of this, Bridge and Portfolio don't play as well together as they should.
For instance, if I write metadata to a nikon raw (.nef) file in Bridge, it is written to the .xmp sidecar file. When I look at the same image in Portfolio, I might see the keywords I assigned but no other user-entered metadata appears in Portfolio. I also find Bridge's interface for entering metadata to be better than Portfolio's. This is not such an issue for .tif files, as metadata is imbedded in those files by either program and can be read in the other program without too much difficulty. I think the answer is for Extensis to bring its metadata handling capabilities up to the standard set by Bridge.
Because of this, I have been doing most of my metadata assignment in Bridge and then importing it (where this works) into Portfolio. Unless the next version of Portfolio improves this, I won't be upgrading and will probably abandon it.