Sitemap generator for Google on large sites

My site has more than 400,000 posts. Does the Google Sitemap Generator work correctly for sites of that size? Are there any potential problems with page timeouts, etc?

Which pages are included in the sitemap? Is it just posts, or are the forum / summary pages included too?

How is the sitemap frequency and priority information set?

Does having a sitemap really help get any more pages into the Google index? It seems like their crawler is pretty good at finding pages on its own. Do you have any concrete user feedback in this area?

We have sites much larger than that running it fine. It has a setting where you can add a delay between each sitemap file, to keep your server load from going too high.

You can choose what is included, Form URLs, Thread URLs, Post URLs, etc, along with a lot of other settings.

Yes, having a sitemap is imperative.

So an individual thread with 50,000 posts wouldn't be a problem?

What are the options for how frequency and priority are set?

I've heard several people say that a sitemap is "imperative", but I'm not sure if that's a result of Google propaganda or real-life experience. I've never heard any concrete feedback on the before/after results of adding one to a large site. Do you have any stories from your customers that you might be able to pass along?

First, I would never allow any thread to have 50,000 posts on it. Even Jelsoft suggests against letting them get that large. It will be bad for system performance. But, it shouldn't be a problem for the sitemap generator as long as your server can handle it.

I don't work for Google, so it can't be propoganda. I have first hand experience with many sites where a sitemap has made a huge difference in how fast, and how well, a site was reindexed. Why would you want Google to just randomly wander around your site trying to find things when you can give it a roadmap so that it can directly start indexing the "good stuff"?