Pros
I was a trading assistant/trader at Five Rings for about a year, and I left because I got the opportunity to work on some completely unrelated problems I'm pretty passionate about. My time there was pretty exceptional--having gone through school at the top of my class, it was a pretty amazing opportunity to be at Five Rings surrounded by people smarter and more knowledgable than me. The staff is a cast of people who were at the top of their class at all of the Northeast's finest universities, and everyone there put tons and tons of effort into teaching me everything they could about all things related to trading and not. The smaller size of the firm and the fact that so much of what people work on impacts everything there definitely makes you feel a lot like the firm is a team where everyone has aligned goals and incentives, unlike a lot of trading firms where people are relatively siloed and only get to think about one problem. As a new hire, I got the opportunity to have tremendous impact and say very quickly in a lot of the firm's important decisions/trading strategies, which is unique--even among similar sized trading firms (I have lots of friends at small prop shops) there are a few layers of bureaucracy separating entry level employees from the people at the top. I sat within 10 feet of the firm's founder from my first day, and any ideas I wanted to share were valued as ideas on their own merit. Disagreement and discussion were encouraged. That's pretty amazing. Everyone at the firm is pretty friendly and open to talking about things, and people have a great sense of humor there. There's catered lunch every day (DOS TOROS DAYS ROCKED) and an awesome pantry. There also was a pretty epic NFL betting pool and an even more epic Secret Santa last year that was really fun. I became/continue to be friends with everyone I worked with there.
Cons
Although this doesn't really apply to me, not everyone is cut out to or wants to be a trader (one of the trading assistants hired with me decided to spend almost all his time doing statistical research instead of trading because of his preferences/skillset). The best traders by definition make lots of bad trades, and some people are not good at being willing to make/admitting to mistakes. Also, if you ever quit from the firm, be aware that it might break your heart--choosing to leave Five Rings broke mine clean in half.