I've been every year. By far, the biggest value is in having the largest concentration of Googlers you'll likely ever encounter as an outsider. For instance, if you're a web developer, you can ask deep questions from people on App Engine, Chrome, and Material Design all within 50 feet of one another. The ability to ask in-person about the internals of an App Engine Pipeline, or get a Chromie to pull up a secret profiling screen to help kill some jank on your Chrome Experiment is why you pay $900 to show up.

(It used to be a lot more affordable, but when people started presuming that they'd seed expensive hardware every year, demand went through the roof.)

If you can get your company to pay for it, go. If you can't, but you are heavily reliant on Google APIs (Android, Chrome, AppEngine, Geo, etc.), you should probably still go. If you work with primarily open tech and are just curious about attending (or if you just want random free shit), it probably isn't worth it.