My previous experience is mostly in R&D startups, doing robotics and natural language processing. I was hired in 2010 and then found out I would be working on YouTube ads. I was disappointed, but decided to try to make the best of it. After three months I realized my lack of interest was going to be a problem, so I talked to my manager about trying to transfer. He discussed it with the site director, and the response was "we don't care" and I couldn't transfer until I'd been there 18 months.
I decided to stick around and see if I could work the system in some way, with the probably naive thought of trying to demonstrate my abilities and catch someone's attention that would help me transfer to a project I'd enjoy where I could make a real contribution.
During Innovation Week (a hackathon) I led a team of three other engineers working on an idea I came up with, and we won the "Most Innovative" award. The other engineers decided they all wanted to devote their 20% time to working on my idea.
My Tech Lead and my manager had no interest in my Innovation Week project, and I still had no way out of YouTube ads. Unsurprisingly my performance on my 100% project wasn't great and even if I made it to 18 months it seemed unlikely that I'd be able to transfer. I left after 17 months at Google.
I've mentioned this story before, and I hope I'm not just grinding an axe--I'm just telling my experience in the hope that it will inform engineers about possible outcomes of working for Google. I am fully responsible for my experience there, but I can say the priorities of the (large, heterogeneous) company were not what I (again, probably naively) expected.