Pros
Learning from and working with every day from people at the top of their field. It keeps you sharp because you have to prove your worth every project. It has a lasting effect on how you do things in a positive way. The quality of life is top notch, with great insurance package, massages, haircuts, workout center etc. You learn quick that if you think about how your work will impact the customer, you will get a better buy-in for your ideas from your peers. As an artist, you learn that this is a great skill to have.
Cons
Valve is essentially a popularity contest, where you will do well if you have lots of friends and prove yourself to be intelligent and do good work. It's a sink or swim feeling most of the time, coupled with hive-mind thinking. After work drinks are rare, as people are precious about their reputations. This causes a feeling of paranoia and formality. Friendships are sometimes based on trust and who is worthwhile to be allys with. It's a shame, because this can put a damper on free-flowing creative ideas. Valve is mostly about execution, not new ideas (as seen in the absence of new IPs). But if you follow the Valve methodology, keep doing high quality work, and don't rock the boat, you can maintain your job for many years.