I've been using sitemap once with a hope to fix my Google ranking.
So I made a sitemap listing with the most important URLs of my site.
Then I was waiting for a month to get the results, nothing really happened.
I searched for the similar situations over the different forums, and found the case when one guy removed his sitemap and that helped him to fix his ranking.
So I did the same. Removing my sitemap didn't help me either. But here is the funny part, in a short time I checked Google statistic tool and noticed - there are references to my URLs listed in SITEMAP (no matter that the sitemap was already removed). That's funny isn't it?
I guess Google is very slow system in terms of applying webmaster's changes to their sites. That's why it's hard to figure it out if the sitemap really helps to improve the ranking. But now I know for sure - it is recognizable by Google.
I was working to improve my Google rank by fixing my other problems, such as duplicates, relevancy, etc. That helped, but not the sitemap.
That is only guess. But who knows, if there is everything right in your site alone with the right sitemap, that might be exactly what we need for our sites
Have a great day