Pros
Intellectual Freedom and Support - Anyone has the opportunity to propose new research projects, regardless of how crazy they may be, and potentially receive (large) funding from a wide range of sources. Colleagues at all levels are extremely supportive both intellectually and logistically. Doesn't matter if you are brand new or a veteran scientist/engineer, the opportunities are everywhere and you'll find no shortage of people to both encourage, advise and support, both informally and through formal programs.
Financial Freedom - The lab is well funded and well equipped, You can probably find what you need to try what you want and if not, there are many opportunities to get funding for such, most of which are easy and approachable, even for a new hire.
Personal Growth and Fun - The lab has superb financial support for those who want to learn more about an area professionally, become professionally certified in specific skills or get a higher degree, even from MIT. No other lab in the country has such a breadth and depth of financial and logistical support programs for its staff growth..
Fun - Smaller Scala Darpa Grand Challenges and annual 'Try out your crazy idea and show it to the entire lab' competitions abound. They are incredibly fun but are as difficult as you'd like. Strap a laser radar to a dog, fly in a helicopter, program a remote controlled submarine, cure all viruses in rats. Where else are you going to find this?
Resume - The lab's street cred in outside institutions, even those outside the defense industry, is absolutely top tier. I heard it was true when I worked there, but it wasn't until I left that I believed it. Lincoln's legit.
Responsibility - Despite all the fun and innovation, the lab's commitment to national security issues is its first, second and third priority. This is engrained and non-negotiable. Despite what you may have heard in other places, research is hard, development is harder. You have to be committed to the mission. You have to take pride in your work. If you don't have it when you get there, you'll have it when you leave.
Cons
Number of people - Lincoln can't grow without bound. The people that work there are great, but there are relatively few of them. This means you need to wear many hats and sometimes you'll be asked to work longer or harder than mortals would consider reasonable.
Ladder Opacity - Rank is important. How good you are at your job depends not only what your skills are but how you apply them. I didn't know this until I left, but know the following:
Assistant staff (leaf)- Be great at whatever specific skill you were hired for. Grow your skill set in that area using the Lab's plentiful resources to become a subject matter expert. Focus on extreme competence in your skill.
Associate staff (twig)- Be the best subject matter expert you can be. Be competent in your core skills, but know more about the area than anyone else.
Technical staff (branch)- Broaden your knowledge past your subject matter and know everything you can about that branch of research and the many areas it touches.
The rank (promotion/compensation) ladder is a zero sum game, if you move up, someone else moves down. This is surprisingly non-competitive, as it is about how good you are at what you do best, relative to how good someone else is at what they do best. If you don't know whether to focus or broaden in advance, it is hard to navigate. The pull by your colleagues in both directions can be disorienting.
