Pros
Google was founded based on the ideals of having a comfortable work environment, and it shows. Free food and tons and tons of benefits are available. Other examples include free bikes, on-site gyms and massages, free shuttle busses around the bay area, and even free co-op cars for employees who bike to work. Aside from the benefits, the people at Google are also what makes it stand out: you'll constantly be in close proximity to some of the smartest people (albeit some are weird) around. Of course since there are so many projects, the work quality can vary, but I was extremely happy with my assigned work when I was there as an Intern.
Cons
Since there are so make projects going on a Google, you may be hired and work on something not so interesting. Of course there are opportunities to change your team, it may take a while. Similarly, since some projects are just experimental projects, it's possible that your project is shut down and you're moved to something else. Though not technically a 'downside', the only reason I am not returning to Google is because of the lack of hiring right now. I was interviewed and offered a full-time job, however it wasn't in the type of work that I wanted to do. This is also the case with a handful of other interns that I know.
