Pros
Very laid back and super relaxed work environment. Alot of exciting science experiments going on all the time. Very friendly nice people work at SLAC. Its probably one of the easiest places you'll ever work as far as pressure, deadlines, etc... Although much slower than other places, you will learn quite a bit about vacuum design.
Cons
If you are the type that makes a comment when you drive by a construction site because you see one guy digging a hole and 5 guys watching, then don't apply. Welcome to that construction site. There are SO many people in SLAC that tremendously over analyse every thing you design that you are not really designing anything. You are basically just a CAD jockey. They discuss to death the size of a radius or red tone of a safety cover, etc... Its actually quite amusing and that's how you have to deal with it or you will lose your mind. SLAC worries so much about Stanford's reputation that it often gets in the way of very simple decisions in the real world. Anyway, if you like the country club, slow underchallenged pace where you get treated very well, gets lots of holidays and breaks, then this is the place for you.