Pros
There are many opportunities in the company to grow to a point, and opportunities to move laterally to find where you best fit in the company. Projects are exciting, clientele is well-known, lots of material to learn, and co-workers - depending on what department you are in - can be great. The choice is yours, if you don't seek much movement, then you are more than free to stay where you are - plenty there also that don't mind just accepting just mere job stability.
Cons
There are opportunities to grow and take leadership because - more often than not - so many people drop the ball, especially upper management. So while you may find yourself taking leadership opportunities, don't be surprised to find yourself alone in those positions with no one backing you up and without fair compensation for the hours of work you will add for yourself. It's exciting, yes, but with excitement comes long hours of work where you are expected to be on-call 24/7, often non-stop at your computer and phone for 48 hours at a given time. So don't expect much work-life balance here if you have an inkling of ambition as a leader. Especially if you are the one setting the rules and procedures. There is extreme lack of clarity of goals and directives from upper management and zero communication. You will often find yourself thrown under the bus, blamed, and your work stolen by not just co-workers but management itself (or their "consultants"). Finally, while you may rapidly accelerate to higher positions - it comes to a point. Once you find yourself competing with upper management, you will realize that if you were basically not a founding member or were not there before the company was even founded, then there is little possibility of reaching the next level.