MikeNoLastName - 9:01 am on Feb 20, 2012 (gmt 0)
If you're talking a "user-navigational" sitemap.htm, which links out to many pages and gets linked TO by many pages on your site for navigational purposes, I just recently removed ours and feel it has improved things a lot. Reason being I realized it was getting a lot of low quality internal backlinks and thus a high PR, but why would anyone ever search for "YourSite Sitemap"? Better to let that PR flow directly to the other closer related pages on your site.
Now if you are talking about a sitemap.xml intended for search engines, I consider it invaluable, particularly if you have a lot of pages being changed, added or updated weekly. I have found that it encourages frequently updated pages to get recrawled a lot quicker, and new pages are found usually within a day or two max. Google has (had?) a routine that you can run manually or have your server OS run periodically which will entirely rebuild your sitemap.xml FOR you in seconds by "Walking" your directory structure according to rules you set, so there is no real manual labor involved in keeping it up to date.