I've tried Combine PDFs, PDF Lab, and PDFJoiner -- and for me, PDFJoiner is the best of the three. Unlike the other two, it creates searchable PDFs (in which Preview's "find" command works). I regard that as essential -- thus I'm keeping PDFJoiner and discarding the other two.

I'm giving it four points rather than five because the user interface makes it a pain to specify what files to concatenate. I can't drag-and-drop files from the finder, and the "add" command always opens in the desktop folder rather than going to the last-previously-used folder.