When Google Sitemaps was released the forums had both positive and negative things to say about it. Some said Google was lazy and wanted a lower costs, more efficient method to find new pages on the Web but others praised Google for offering a solution of indexing hard to reach pages. All in all, I am happy that Google released this type of free inclusion program. Why? Well, according to some (see the threads linked to here) Google Sitemaps has increased their visibility and traffic.
Kim, Cre8PC, wrote a guest article here last week named Google Sitemap - Is there a real benefit? and links to a thread at Cr8asiteForums named Effect of new Google Site Map on SEO? As a follow up, Umit posted his experience with it. He said, "I think it is beneficial for especially forum sites." He adds that "my forums' hit increased nearly %25 more."
Now there can be many reasons for this. Google Sitemaps should help those sites that have dynamic looking URLs get indexed quicker. It should not improve rankings of a site (in theory) if the site is already getting indexed quickly. No matter how "deep" the pages might be, if you have a search engine friendly site internal linking methodology, with search friendly URLs and so on, you should not need Google Sitemaps. It is hard for me to make generalizations on this topic without actually partaking in such an experiment. So I think I will partake soon.