If I work for Big4, I will have great opportunity to work with various clients. It provides a solid foundation in accounting career. I just wonder why many people leave the firms after working only 3 - 5 years? What do they do? What is the best career path after leaving the firm and get into industry?
I am an international student. Sorry for my English anyway. ^^
The other comments make good points, but I'll add what I can.
Many people start in the Big 4 because of the excellent reputation of the firms, and the experience of working with top professionals and access to high-profile clients. Also, it's one of the easiest ways to ensure your CPA license gets taken care of early on. But the Big 4 typically demand very long hours and hard work, plus as you move up in the ranks there is pressure to bring in new clients, and many people don't want to get involved in that salesman-like pressure.
Many people who start at the Big 4 leave to take jobs in the private sector, sometimes with one of their clients. These jobs (usually) offer better compensation and fewer hours required. Also, the perk of not having to commute to various clients is nice.
Obviously, the "best career path" question is purely subjective, and depends on what you're looking for out of your work. Individuals interested in auditing would probably gain the best experience from Big 4 or similarly-sized public accounting firms. Tax-oriented people could really go either public or private with their career. A CPA license is not necessary for people interested in private sector work as an internal accountant or controller, but it typically will help you gain a better job.