Pros:
1. Hack out awesome stuff wicked fast. No design reviews, no cross-function meetings, rubber stamp code reviews, and no PMs wavering between several mutually exclusive ideas. It's a group of engineers with one goal: getting something done.
2. Creativity. It gives you a chance to approach problems you might not normally encounter on your current project. For instance, an infrastructure engineer might work on a product idea- and bring with him/her a fresh perspective. Some of my projects have included cryptography, streaming video, and a rating system for bughouse.
3. Camaraderie*: There's a reason some fraternities make pledges spend sleepless nights together: it fosters a sense of community. People get to know each other, they cross-pollenate more than usual.
Cons:
1. Losing two days of work on your normal project. Bonuses, raises, and promotions aren't based on a cool hackathon project; but on sustained output with your normal work. Sometimes it's just impractical to lose two days of work for a night of hacking.
2. It feels more contrived as a company grows: Big companies just can't pull off an authentic hackathon. Hackathons end up taking on multiple purposes (themes, recruiting, public relations, even launch parties), all of which detract from the main idea: hacking (where "hack" is used in the sense of exploratory programming, not its alternate meaning as a reference to breaching computer security.) Plus a relatively smaller percentage of engineers will actually participate as a company grows.
3. It can be hard to focus. Between a few kegs, ping pong, poker, and socializing; it can be hard to actually accomplish anything substantive. Once a hackathon has taken on a "party" atmosphere, it can be impossibly hard to reemphasize hacking.
Overall conclusion:
Hackathons are awesome, yet increasingly difficult to effectively organize as a company grows
Camaraderie means mutual trust and friendship among people who spend a lot of time together.