yes i am using this Google Webmaster tools since 2007 it's very useful to know your site status. like from where your sites gets back links, errors in sitemap.xml, internal links, external links, Google crawler staying time on your sites etc etc.
What I find most useful is the information about what keywords google finds in my content and in links to my site. I compare this to my ranking in the SERPs to see where I might be able to get a boost from a little link building.
Google Webmaster Tools has become something like inevitable for many webmasters these days, as some unique information are provided that are very great and updated regularly.
I have been using the Google's Webmaster Tool from last 2years. Mostly, the crawl error feature has helped me to improve the site performance and the user experience on my site. It also show you the details of the people who has subscribed your feeds.
now google webmaster tool with new look is very helpful, especially if there's a problem with site that need to make contact with google like re-inclusion etc, fast response (in 2 days) from them.
Our websites have benefited so much from Google so I think it would be a terrible idea not to work as close with the worlds biggest search engine as you possibly could. Those tools are very helpful in showing you what Google sees and it can show you a lot that you wouldn't have found on your own.