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Yes. For the first few years, I was amazed at how far people would travel for it. (They still do, I'm just used to it now.)

Indeed. There are plenty from a lot of countries, even to places where there are not Google offices there.

My company pays for the trip. This makes it even more unfortunate if you don't get a ticket: It not only costs you a good conference, but also a free trip to san francisco. ;)

Visa hassle and trip cost could be off putting considering you could watch all the videos at home.

there is no "visa hassle" if you come from Europe, and there are no trip cost since the company pays for everything.

While the sessions are available as video streams, the sessions and discussions in between the sessions are not. I was at I/O last year and I not only had a good time, I also learned a lot during the three days.