I recently wrote a sitemap gem
for one of the more larger sites
I have been working on, which was recently revamped.
To be fair I searched for existing solutions, but I found them lacking for one reason or another. One of the main problems that these plugins couldn't overcome was generating a different url (such as dynamic domains) based on an existing attribute.
There are some limitations as well, most notably the 50.000 urls per sitemap, but I'm planning on resolving this issue some time in the near future.
If you decide to test it I strongly believe that the docs cover the functionality pretty well. Additionally make sure using webmaster-tools that google bot can use your feed.