I got to apply what I have learned in a way that made me responsible for what I create (more so than when you are just working on a project for a grade). I've picked up two languages and a new design pattern. My project wasn't just something to entertain an intern, but is something that will actually be used. I got to be responsible for every aspect of it, and learned the process of software engineering from the infancy (project description) stage, all the way to testing.
I learned a lot in twelve weeks, and had a great time. My mentor was fantastic at knowing how to teach. When I was wide-eyed terrified he offered help and guidance; when I (thought I) knew what I was doing, he would let me go off on my own and get things done. The rest of the team was amazingly supportive and eager to help when I had a specific question in an area they were known to excel in, going as far as to take time out of a really busy day to sit and work through a problem or bug.
This is minor, but I think a little more help finding short term housing would be a good idea. I ended up having to search on Craigslist, which is a bit scary.
Advice to Potential Interns
Calm down (don't be nervous). Everyone knows that you are here to learn, and the only wrong thing to do here is to NOT ask questions. Don't understand your project description immediately? Ask questions. Your mentor and fellow team members are amazing resources of knowledge.
Get involved! Seriously. Go to lunch with the group of interns that go out once a week. Go out on the weekend trips and the crazy adventures. Spend off hours times with the people on your team, and got o lunch with them. Join a club or two (volleyball is amazing). Get to know your mentor. Go to the newbie lunch on Wednesdays to meet other employees. Seriously, this internship can be AMAZING, but only if you let it.