I just added a more complete sitemap to our site, which includes a forum with over 80k threads. Obviously I did not make it manually. I created a script to do it that updates on a daily basis.
The main benefit for me is that through Webmaster tools I can now clearly see the number of urls that I would like to have indexed.
Prior to this we just had a sitemap with our main static urls. I don't think it had any negative effect.