Pros
The sky's the limit. This is a company that you can come to and build your own practice from the ground up. If you convince clients to hire you for huge projects, you'll do well and rise to the top rapidly. No one is there to stop you. You can choose to walk all over people to get there or not; there are many paths to Principal. People are generally good, and generally bright. There are a few phonies and bull crap artists, but the company overall has a very good reputation among clients and outsiders - and for good reason. Most interview candidates come armed with almost the exact same phrase: "I'm interested in Exponent because I like variety." You will certainly get that here.
Cons
Biggest con is a lack of management. The act of "good management" is not rewarded in any way. The term "manager" is thrown around as if giving out candy. Make a lot of money for Exponent? You're a manager / senior manager / principal - never mind if you never actually manage anything or anyone. Really good at training people, hiring people, or running a lab? You get a pat on the back at best, and at worst your bonus will suffer. The Employee Stock Purchase Plan is a complete joke (5% discount off of market value, with no control over the date of purchase). Stock awards / options are for Principal level and higher only. At that level, the options factor in to your bonus calculation. Bonuses are heavily skewed towards the rainmakers. Bonuses at lower levels are often a joke, unless you are the top performer in your pay grade.