Pros
My work is a lot of fun - sometimes I can't believe they pay me to do it. A lot of good dedicated people to work with. In my case, flexible schedule
Cons
As others have said, avenues for professional development/advancement depend entirely on your manager. Science is the most fun job, but has the least potential for advancement. Ph.D.s are valued and compensated greater than those with greater experience. One of the biggest negatives for employees is that, as you get more years with TNC, you are more likely to get laid off. As the economy cycles, so does TNC, so periodic big layoffs are endemic. In my chapter (state), layoffs are entirely arbitrary. The first to go are those with greater seniority because of their higher salary - I have seen many great workers, with more than 15 years, get laid off - the de facto result is that TNC discriminates against older workers, TNC's mission has changed drastically in the past few years. Where we once were about saving "the last of the best and the best of the rest", we are now about protecting those areas that resonate most with people, rather than those areas with the greatest conservation value.