I've tried Combine PDFs, PDFLab, and PDFJoiner. Of the three, I think Combine PDFs is the weakest. It PDFs are about 50% larger the other two, and they aren't searchable (e.g., Preview's "find" command won't work).

For me, it's essential to have searchable PDFs, and PDFJoiner is the only one of the three that can do that. Thus I'm using PDFJoiner even though its interface is a bit clunky.