I've worked at Google as both a contractor and a full-time employee. What it's like should be divided into two categories: official policies, and your actual perception. The latter depends a lot on your team; in mine, all but the PM are contractors, so there's no discrimination or anything. Here are Google's official policies regarding contractors:

• Your badge is red and has a 'C' on it
• You're specifically instructed to NOT say on your social media profiles that you work for Google. You work for <company xyz> AT Google. This may confuse some folks and has less of the ring of "I work at Google".
• Some rare areas of the Googleplex may not be open to contractors. Not sure which. Haven't seen anything restricted yet that I wanted to check out.
• No access to the corporate wiki, which tends to have old stuff anyway. New stuff goes on Google Sites. You can also request access to the corp wiki if you need information stored there, which is easy to justify as various programming guidelines/styles are on it.
• You have to pay a nominal fee ($1.50) to be able to use shuttles, so the IRS doesn't consider them employee benefits. But other than that, cafeterias, snacks, gyms, showers etc. are free to all
• You have to fill in a timesheet for your employer. This is mostly a formality.
• Hardware is second tier for contractors. E.g. used/refurbished laptop, previous gen. desktops
• You also get only one desktop OR laptop as a contractor. Employees easily get both. However, if your manager and their director approve your request for an extra machine, you're good. Without a corporate laptop, you can't access the internal network unless you're on Google's campus Wi-Fi, which surprisingly doesn't include the one in shuttles.
• You can't officially use many educational opportunities ("Grow"), e.g. instructor classes. Presumably if you show up to audit something, you won't be kicked out though.
• + some related tools, like FreshLens (giving feedback as an outsider to Google Products, when you're new to Google), are limited to employees
• Shouldn't bring in guests. Employees can, and delight the guests with Google's free food.
• Some special occasion perks (I imagine most) are limited to employees. One example was the BMW Drive Now program, which has a membership special for Googlers.
• A bunch of events are for employees only. Others are specifically open to contractors (e.g. Movie nights). Others don't specify, and probably nobody will mind (e.g. speakers or fitness instructors of all sorts).

However, you still have access to on-site facilities (cafeterias, gyms, mail, laundry, bike repair or whatnot). Anyway, as a contractor, you're probably (hopefully) well-paid, so do your job and stop looking for stuff to mooch :)

One odd pro about working as a contract is that you don’t have to go through the performance review cycle. Some have complained that this perf process has caused some perverse incentives at Google: teams or employees launch new products mainly to get promoted, or people who deserve promotions don’t get them for unclear reasons.

Another one is that you probably will be able to focus on one thing and do that well. Some other roles have to deal with a variety of technologies and they end up knowing a bit about a lot, but lack deep expertise on any topic.

Overall, I haven't had problems as a contractor (save for a snag re. requesting a laptop). Tickets that I filed were generally treated as seriously as I expected those submitted by an employees would be.

Note that I was contracting through E-Touch, a staffing agency. I don’t know what it’s like to contract as yourself.