Very easy to use, with an excellent interface. However, the file it produces does not seem to be a .pdf file, is listed as "Document" with Get Info, and seems to be openable in its entirety only by Adobe Reader. Preview can not open the merged file. Under Finder/Open With, mainly graphic programs are suggested, and they open only the first page.

I'm going to stick with PDFCombine, which does output a pdf file openable in Preview and is also very easy to use.