Another current FES student here. What castille89 mentioned above is pretty spot on. The part about FES presenting a good outside image is very true - one of the things that convinced me to join the program was that a lot of people (even outside my field) knew about the program and regarded it as one of the best in the field. But these people did not attend FES. The folks I spoke to who did go to FES didn't sound disappointed par se, but they did express that the program did not seem practitioner enough, or that the degree did not lead to a substantial increase in salary. Personally, while there are some good things about the program, I am for the most part disappointed, so I just wanted to share some of my thoughts so far.
Others have mentioned that the students tend to be stereotypical hippie environmentalists/freewheelers - this assessment is accurate. If this isn't your style, it can be difficult to feel comfortable here. This year, FES added structure to their curriculum. While in the long run, I think this will be a good thing, the school is dealing with a lot of challenges with implementing the curriculum. Students are forced to take foundations courses that a lot of people feel are a complete waste of time. Granted, this is the first year they are offering it, so it will take time to work out some of the challenges. Furthermore, the student to faculty ratio is not great - there aren't a lot of professors in certain subfields, and it can be difficult "standing out" to professors and really leveraging that network. Also, many students complain about the career services, who are not particularly helpful.
Another thing to note is that Yale is very much an undergrad focused institution, and as a grad student you can sometimes feel kind of like an outsider. A disproportionate share of the resources are dedicated to the undergrad population. While Yale does have an active grad student community, I have mixed reviews about the grad experience here. Although the FES community is quite socially active, I generally find grad student life here to be a little depressing - New Haven is not a great place to live, and anxiety is fairly rampant among students here. At FES, there is quite a bit of anxiety about getting an expensive degree that does not necessarily lead to job security/a higher salary.
There are some pros to the program, but I don't know if they distinguish FES over other peer programs. The faculty at FES are very impressive, and many are at the top of their fields. I think where FES truly stands out is the ability to give you perspective into how things work in the policy/business realm. It's not a hard skills school, but rather a place where you get to hear practitioners share their perspective from working at a high level. I'm also very impressed by my peers, and the background they come from. You'll find many people who have worked at the top environmental organizations. All of this is to say, FES is not really a school for technical skills, but rather a place that expands your perspective, both through the professors and your peers.
One final word of warning: definitely consider your financial circumstances and the ROI on the degree. Yes, it's a Yale degree, but one where the average starting salary is 60K. So just something to keep in mind.