Pros
I have been working at Arm for a month now. In the group I work for, the project structure is set up as cross-site teams, so although I am not working at the main Cambridge site, the same opportunities that would come from working in a large site are available to employees in the smaller offices. This is not only my impression, but was highlighted in an introductory meeting by senior management. This is really important to me. It is in contrast to other places I have worked in in the past, where individual sites had their own teams and identities, which can lead to more of an "us and them" mindset, and can also make it feel like opportunities are limited. I attended the Cambridge office recently, and I was made to feel very welcome. I was also fortunate enough to take part in the IPO celebrations, which were just amazing! My early impression is that Arm has a great culture. There are several groups in the chat tool representing different interests, and in general people are helpful in answering questions. It feels very much like a culture of openness. There is plenty of training material available to help with learning. The company is also being realistic in allowing a sensible time for new starters to ramp up on projects.
Cons
It is early days, so I can't think of any cons so far.