Pros
Leviathan Security was an amazing experience for me. Everyone there was dedicated to the betterment of our customers' security. When I had a problem (and let me tell you I had more than a few) there were plenty of people with experience and intelligence that could help me work toward a solution. Everyone shared in victories and setbacks together. The company is owned by employees which inspires confidence in how it is run. That's not to say that everyone got their way or that everyone's opinion was valued as much as the C-level executives, but that the management took responsibility for their actions. This is a far cry from what I have read and seen elsewhere. Hours at Leviathan were easier than most competing firms because almost everyone had a family they wanted to go home to. Similarly, Leviathan didn't send their consultants to the client's worksite unnecessarily, travel was less than 25%, much less for employees who wanted to stay in Seattle. All of Leviathan's management were and still are excellent hackers. Taking orders from someone is a lot easier when you respect their skills and can fall back on their experience. Despite everyone's concerns, Leviathan is fiscally very conservative which means that everyone gets paid and the business isn't going to get sold to NCC Group any time soon. Leviathan's IP policy is the most liberal that I've found. Read it yourself. Leviathan encourages their employees to work on their side projects when they aren't fully utilized. Utilization was below 75% for consultants when I was there, which meant plenty of time for research. Researchers and managers were at around 100% utilization though unfortunately.
Cons
Leviathan's business side has always been a major issue for practically all former employees. I saw the lack of a mission statement as a mission statement itself: everyone should work toward their goals and benefit one another in the process. Others not so much. Because Leviathan is owned by employees, they are not very extravagant about their expenses. This is good for customers and good for everyone's salary, but celebrations were not as crazy as people expect startups to be. The turnover rate in the industry is fast and Leviathan doesn't have a silver bullet for it. Good people come and go. Leviathan's Mutual NDA can feel like a gag, but if you work in the industry you understand opsec well enough to expect it. The whole industry is worse off because of it. Security work for customers can sometime feel like a Sisyphean task with few victories and many shrugs. This can hurt morale. Leviathan does work for hire, which means any meaningful piece of software written during billable time is forfeit. Leviathan rarely attempts to convince customers that it is in their benefit to save valuable work. If you care about the software you produce, you are obligated to set it free. Obviously this is not true of research, only consulting.