Pros
A center of interesting, world-class, cutting-edge research. Colleagues are world experts in their fields and generally a pleasure to work with. A great place to learn new things and to flesh out new ideas.
Cons
Management is a bit too risk-averse and short-sighted. Lots of good ideas are not pursued or funded for this reason. Employment is much less secure than it used to be. Starting about 2010, lab has quietly shifted from a very secure "just cause" employment status to a much less secure "at will" status. Job security is now no better than most Silicon Valley companies (lab management apparently wants to increase turnover to be in line with industry jobs). But SLAC salaries are lower than industry. Salaries will have to increase if employees are to be kept. There is a general sense that the Lab does not value employees as it once did.