It’s OK, but not optimal.

It does what it says just fine. You know what would make this app amazing and 5 stars for me? 2 things: Preview window and ability to drop any image file onto the list and convert to PDF.
There’s another app out there that will let you drop PDFs, JPGs, PNGs, almost any kind of image file and output it all to a PDF.
Wish this app did this. For now, you need to convert everything to PDF (preview works for this) then bring it into this app.
Still, strictly for merging PDFs, it works quite well… even without a preview window.

benster74 about MergePDF : Combine PDF files