Pros
Work - The projects are great, and absolutely relevant to the company. Needless to say, interns aren't assigned "toy projects", and instead get challenging and stimulating projects.
Learning - There's a lot to learn on the job, and the mentors really help in easing the interns into the code base. That being said, the employees are really approachable whenever there's any help required.
Environment - The timings are flexible but there is pressure of deadlines. The overall atmosphere is pretty friendly, and the office is quite progressive.
Cons
This would really be called nitpicking, but the compensation paid to the interns is perhaps a little on the low side. My take would be that the quality of work more than compensates for that though.