The cons are flipped versions of the pros: Google hires very smart people and then puts them in fairly mundane roles. They used to try to take your preferences into account when allocating; they seem to do much less of this as they've grown. If you're at all purpose-driven, you'll eventually become restless at work, looking for something with more meaning than the project you're on. You could potentially 20% something, but that notion has always applied more to some groups than others, and the company has tended to downplay it in recent years.
More disturbingly, there's a severe opportunity cost to your side projects: their position is that everything that you do in your personal time belongs to them, although that is not what the employment agreement says and would be an unenforceable position in California. They instead get around this by suggesting that everything that touches the web or mobile (and perhaps the desktop as well) is competitive with them. There's a committee that will examine ideas, but it appears to be moribund.
Google has grown tremendously, and systematized to the point where it's a large machine, needing an increasing number of cogs to keep the engine running. The culture has been eroding recently because the company hasn't been able to reconcile it with its growth, and because the company has sidetracked from its core mission and thrown resources at "me-too" projects such as Google+. I predict that this will hurt the company's outlook in the long term, as most of its revenue generation still comes from relatively few core activities which are exposed to market and competitive risks. As it becomes less entrepreneurial, it becomes less able to diversify into new areas, and thus becomes less resilient. I would expect this to take 5-10 years to become apparent (the market would need to shift in a way that causes one of their pillars to collapse), which means that it may not be an issue for most people considering it now.
One other thing I've found is that while they care collectively about their engineers a great deal, they're generally not very willing to go out on a limb to make individuals happy. Food and facilities people are the exception, as they do often take engineers' feedback into account.
