Pros
The people who work at Arm are very switched on and hard working, which makes it a vibrant technical community to work in. The emphasis on trust and partnership in the team (as well as the unbelievable benefits package) makes it clear that the people are the assets to the business. The effect of this is that the team's opinion is truly valued, which creates a great atmosphere to work in. If there are genuine needs for training & support, these are usually granted really quickly. If there are specific requests for career growth and experience, these are usually granted wherever possible (even in small ways). All of this adds up to make an empowered team which feels appreciated and performs well
Cons
The end product being worked on isn't always visible, which makes it harder to explain what you do to friends and family. Being a foundation point of a whole industry is incredibly important, but not always the most flashy thing to do.