This is a fine application, to the best of my knowledge the only one that converts HTML directly to PDF. It's a sophisticated application which requires some close attention to use -- and that's only fair, given what it does. I think Michael Sweet has done an outstanding job.
Pros: Quick. Clever design (using HTML comments for things like page breaks). Versatile -- conversion can be controlled from the command-line or from within the HTML document. It *is* available as a true GPL product and as a commercial product -- I use the GPL version.
Cons: Table width is a tricky business, I think because PDF must be aware of the available width of a table, while it doesn't matter in HTML. Can't insert dynamic information into headers/footers yet (promised in 1.9, I believe, and eagerly awaited by this developer). Odd behavior of certain ancient tags, like _em_ and _strong_. However, these can be corrected by the script into which HTMLdoc is embedded.
A final note: The need for the environment variable HTMLDOC_NOCGI=yes is poorly documented. Without this, you won't be able to get HTMLdoc to work in a shell script.
Conclusion: An outstanding product.