Pros
Very stimulating and challenging work in a variety of defense oriented fields. Incredible benefits package, and excellent opportunities for furthering your education, funded by the lab. Great leadership, patience, and concern from higher ups provides the opportunity to grow and learn much more about a field. Opportunity to work on 2-3 projects at a time, provided the ability to multitask, which allows for further growth and provides an opportunity to network with coworkers. Lastly, a lot of cool programs and clubs and intramural sports offered at this campus of over 4000, including basketball, softball, an on site gym, and other amenities.
Cons
Although they have only gone through one downsizing since WWII, all work is heavily defense funded and national defense oriented. Potential to be affected by cuts in defense spending or transitions in defense priorities. As an intern, your choices of projects can be, but aren't necessarily, limited. There aren't a lot of downsides in general, as APL is very generous towards its employees in my experience.