G Sitemap - Beneficial or Not?

Is posting a google sitemap beneficial or not?

We have released new pages, all with new url's, so the old pages are getting the traffic, but we want the new ones to get the traffic, 301 redirects are quite frankly impossible with the amount of pages we have (500,000+).

All our traffic comes from Google, so if it's not beneficial we won't post a new sitemap.

What are the thoughts?!?

I'd definitely set up a sitemap. The larger your site, the more beneficial it is, but I'd say in your case it is definitely worthwhile.

We set-up a sitemap before, and it seemed like it had a detrimental effect. Has anyone else experienced this?

From Google's FAQ:

"7. Will participating in this program change my pages' ranking in Google search results?

No. Using Google Sitemaps will not influence your PageRank; there will be no change in how we calculate the ranking of your pages."

Unless you set up your sitemap completely incorrectly, I don't think there can have been a negative effect. We recently saw one of our dynamic sites go from having 800 pages indexed at the start of January to having over 1,000,000 indexed by the end of January. Top stuff.

So far using the sitemaps feature has helped a lot for some of my larger sites... finally some of my old pages are de-indexed, all my new stuff is listed and I've discovered some errors which have helped me clean up a lot of silly mistakes (404 headers etc) as a direct result of the "errors" listed in the sitemaps info.

I have nothing but good things to say about it.