Pros
I worked at Jane Street for a three month trial as an experienced hire, doing research on a trading desk. Jane Street is an awesome firm. It's the most exciting environment I've worked in — being on the trading floor feels like you're at the nexus of markets. And looking ahead, I expect the business will continue its trajectory given the growth of ETFs and the electronification of new markets, The talent density is very high — people are all extremely sharp. It's more diverse than most finance firms. The hierarchy is remarkably flat — everyone sits together — it's not obvious who is senior or completely new. There's a decent amount of open discussion of proprietary ideas (more than most HFs, less than most tech firms). Progression seems to be extremely fast for those who contribute. The working day is short and intense. Almost everyone is in by 9 and most people have left by 6. During the day, people are focused — no one is reading long form journalism online, or popping out for lunch. For those who care about this sort of thing: there's every type of lunch / snacks / drinks you could want. I ate way too many mini-toblerones.
Cons
A lot of the technology is build in-house. That's both good and bad — e.g. the code review system is a novel take on how code review should work, but it's also slow and primarily in emacs. The firm moves quickly. People are a bit less reflective or conceptual than at other places — the focus is on doing good work and moving on. That's also consistent with the sort of work you do — there's less careful methodical research than at, say, a quant fund. (That said, there are cool tech talks / regular speakers, so it's not as narrow as that might sound.) Feedback & accountability is stronger within teams than across departments. Partly because there's limited structure, I got the sense there's a small group on each desk that drives the business forward, and it's not completely clear whether you'll be part of that group or not. This is more of a concern for experienced hires than those out of college.