Pros
You interact with great developers, original thinkers and interesting people all the time. Unlimited munchies are great, three free meals a day (or two in satellite offices) is great, medical benefits, partial subsidy of fitness membership, subsidy for ongoing education (though in reality you are unlikely to have time to use it) are all pluses. Being on the winning team feels good, especially when winning an uphill battle against an entrenched monopolist.
Cons
The days when Google was the coolest place in the world to work are gone. Google is deteriorating at the edges. Many managers at Google got their jobs just by having low employee numbers and are otherwise unqualified. Once entrenched they tend to show little concern for their reports, concerning themselves with "managing up" to their own manager. Google is supposed to have a project matrix where tech leads are peers, not managers, but managers commonly flout this and micromanaging is endemic. Moving between projects is limited by complex procedures and is rarely attempted. In satellite offices the selection of projects to work on is limited and to make matters worse it is discouraged for engineers to work on projects not centered in their own offices. Being friends with your manager is a more effective way to get promoted than showing competence. In fact, showing too much competence or initiative is a good way to earn the ire of your manager. Performance evaluation is supposed to be by peer review but in reality, feedback from peers is ignored and only the manager's rating is taken seriously. Political infighting and character assassination are increasingly the norm at Google. Managers turn a blind eye to it, perhaps because they have found such techniques useful in developing their own careers. Google base compensation is on the low side, and is supposed to be more than made up for by incentive bonuses, but these are largely illusionary because few employees receive the necessary "exceeds expectations" performance evaluation. Managers at Google tend to consider themselves special people, better than engineers. Few will bother to greet or otherwise acknowledge the existence of anybody other than another manager if they pass them in the hall. Except for the weekly TGIF cross-company sessions where the founders candidly answer questions from all employees, management at Google is increasingly secretive about procedures and plans.