This functionality is now in Preview, true, but CombinePDFs is much better for working with large documents. I was just trying to read 5 pages of an 1100 page PDF in Preview (and Skim) and my computer was beachballing every time I tried to move from one page to the next etc. Trying to extract the pages from the PDF using Preview led to similar delays. I pulled the PDF into CombinePDFs and extracted the 5 pages practically instantly, with no delays, & now have a much more manageable PDF to read in Preview. If you're only dealing with short PDFs, maybe you can stick with Preview, but if you want to extract book chapters, articles from collections of journals, etc. CombinePDFs is invaluable.

It is way better than the Preview app in Leopard because the file size keeps low. Preview somehow makes the final PDF rather big in size.

Does it keep the table of contents?

Well, this app has been great to me but it seems like this functionality is in Preview with Leopard now so I may not need it anymore. Drag the mini-icon from one preview instance's title bar to the sidebar of another open preview window and bam, they're merged. Other neat features like this have appeared too.. check macosxhints.com

This is an excellent app - functional and simple to use. This is how everything should work.

Allows you to rearrange the order of the pages, to add jpg to the stack. Stable, with no unnecessary options. I would recommend it over other solutions, even commercial ones.

As magnusvk pointed out the name does not include an apostrophe.

This app is really called "Combine PDFs" not "Combine PDF's" as should be quite obvious from English grammar.