Pros
Great technical work opportunities across a number of projects and products. Lots of cutting edge work across CPU IP, systems IP, architecture, software, etc.
Very collaborative company. It's a place where engineering discussion/debate is common, encouraged, and courteous.
Very open company. Senior management is transparent and forthcoming with information. It's easy to reach out to coworkers on other projects to collaborate and learn.
Engaged community. Volunteering events, employee resource groups, common interest clubs, group discussion channels.
Global. Interesting work at multiple sites in US, UK, Europe, India, Asia. Lots of collaboration across sites.
Work/life balance. Strong culture of balance, despite hard work and aggressive schedules. Sabbaticals, company-wide days-of-care.
Competitive pay, good benefits, great stock ownership opportunities (RSUs, ESPP).
Cons
As is common in the CPU design field, it's common to work more than 40-hour weeks. This typically ebbs and flows with project cycles. It's not a job you can coast through. This is not necessarily a con though. You can't work at a company that's on the cutting edge of technology with great stock value and not expect to work hard. Arm does have good work/life balance in the face of working hard, but expect to work more than 40 hour weeks.