Pros
Location of client's office, my coworkers, equipment provided by client.
Kontras
I was a contractor of a contractor of a public organization. You are supposed to work their schedules and client's schedule and no payment of overtime. The project was weak, no opportunities to grow in company. When I asked for a raise based on productivity they dare to say I'm thankful to have a job and if I don't like it, they actually can open the door and thousands of people would go crazy to occupy my chair.
The best of everything, I was fired months after that conversation (due to economical crisis) and asked me a month later to work with them again for less money. Seems no many people were "going crazy" after my empty chair.