Overall, I love it. The ability to tell the search engines “hey, check this page out often and don’t bother with these over here but once a month” could prove quite useful. That said, part of me wonders if it isn’t just adding extra work for web developers that already have too much to do, though for a proper dynamic site a little work upfront to let the site generate the sitemap itself should mostly negate that. Even so, Google seems to do a pretty good job on their own without our input.
Then of course we all know that (the majority of?) sites will use the sitemaps completely improperly. Does anyone want to bet how many “webmasters” will setup a Google Sitemap telling them to check every single page, every day, with all pages at a priority of 1.0 because everything on my site is important! The concept of ‘relative to other pages on the same site’ will escape them and while they’re only hurting themselves with that, I feel for anyone having to explain to a client why a particular page on their site was only assigned a 0.3 priority let alone 0.1.
In the end I expect it will stick around, Google will ignore a good portion of them due to improper setup, and those with well designed sitemaps will benefit. Which I like to think is a good thing.