Pros
For undergraduates who (like me) are interested in research about applications of machine learning to NLP and computer vision, it is difficult to imagine a better position than the PYI program at AI2. There is a great deal of freedom with respect to research topics, ample computing and data annotation resources, and great access to highly competent senior researchers and even faculty members at leading academic institutions. You are encouraged to publish your research, which is helpful both for graduate admissions and (potentially) for industry jobs. AI2 has many great benefits like other tech employers, including paid conference expenses when you have published your work.
Cons
Perhaps one con is that there is not necessarily a clear path of advancement within AI2 at the conclusion of the PYI program (as far as I can tell). However, the duration of the PYI program is flexible, which is very nice.