Pros
Jane Street has been an incredibly rewarding experience. You are working with some of the top minds in the world, collaborating on very difficult problems. I am a trader, so I am constantly scouring the market for opportunities, tweaking our trading systems, and developing tools to help us trade more effectively. There is very little to no administrative work (outside of doing interviews), and a single hour long meeting per week. The people here are incredibly smart, yet humble. Think of it as the very top of the ivy league math and computer science departments, so probably some of the geekiest people you've ever met will be your co-workers. However, not all departments are like that and some are extremely outgoing and charismatic (dating back to the days of floor trading I guess). Everyone is willing to help on just about anything, from the simplest questions to the advanced problems. Few are arrogant, and in fact the company will not hire you if you have an unchecked ego. The pay is really high at the start, even compared to other trading firms. You will have no trouble securing a comfortable apartment in any part of Manhattan on the base salary alone. The bonuses are also high, and they are based on overall performance of the firm. You don't have an individual profit/loss number and never feel that an unlucky stretch will sink your bonus opportunities. The perks are awesome, I won't go into specifics but you receive a very generous package of benefits covering just about everything you could think of. Breakfast and lunch are catered, or you can also have food delivered on the company tab. The kitchen is fully stocked with food, fruit, beverages, snacks, and candy. Jane Street really takes care of its employees. The hours are also quite reasonable for this kind of job. You'll never have set times that you must come in and leave by. I work about 50 hours per week and am about in the middle. Some people work more, some less. There are also very few to no days where you feel that you "must" stay late to finish something. You never work weekends or nights. The people are awesome. Everyone works hard while they're here, but nobody gets burned out or has unreasonable expectations. There is practically no hierarchy at all, and little to no busy work. Finally, the firm has very high ethical standards. We will intentionally avoid strategies that, while legal, are ethically in a gray area. I've yet to run into anything truly illegal happen at this place.
Cons
The only weird downside is eating lunch at your desk. Traders don't get up from their desk more than a few times a day, and never for longer than to grab food or use the bathroom.