Pros
The culture is great. The people at Dropbox really care about doing right by their users, all the way to the very top, and it shows. You can sit down at a random table at lunch, introduce yourself, and meet some new people. And it's great that Dropbox's products make money by selling the product to the users -- it's not an advertising or data mining business model where there's a strong tension between what the users what and what the customers want. This means that while working at Dropbox, you can really focus on just making the best experience possible on products that are enormously popular. While Dropbox isn't really a startup anymore, the office environment is pretty fun, and it feels like they've done an excellent job of scaling up the business while still keeping a great culture. This wasn't the case a couple years ago, but Dropbox today is very family-friendly, a huge supporter of women in engineering, and an all-around great place to work.
Cons
Dropbox offers less vacation time than many of the other hot companies in the valley (though it offers more than the Oracles and Ciscos of the world).