Vantaggi
The biggest perk of working at Arm is the excellent work/life balance. To put it bluntly, the parking garage empties out after 5pm. Sure if you want to be a superstar and work 60+ hour weeks you can, but no one is going to scold you for going home at 5pm on the nose. Another big perk is the level of impact you are able to have on products. Arm teams are not large, meaning your contributions have direct and measurable effects on the end-products. Managers will readily delegate large responsibilities if you show appetite and aptitude. The last pro I'll mention is the open culture within offices. Products teams have no problem sharing information with you, There are no information silos or secret projects. Ask questions and you'll get answers. There is no mandate to keep quiet about what you're working, unlike some other juicy IC corporations.
Svantaggi
The promotion system, like any large company, is not favorable for existing employees. People with less experience regularly get hired into higher grade levels while existing employee have to "wait their turn" for promotions. It doesn't seem worse than any other large company but it is still worth mentioning.