Pros
I like working at Arm. I don't love it. Arm misses some excitement that you get in other tech companies. It's a fairly nice company when some people in some engineering roles can develop personally. Non engineering parts of Arm are much more dysfunctional. Progression is slow and based almost entirely on luck and time in the role. It's very quiet out here, you don't hear much banter in the office, people who have worked together for years don't know each other. Not much of the little free stuff that makes people happy, even team lunch above £30 pp is not allowed. Don't expect to be yourself at Arm. Arm has very clear definition of what a member of staff should be like. Arm has a lot of interesting projects but few people do the interesting bits in them. If you're new you most certainly won't be one of them. There's an opportunity to learn from some brilliant people and it's relatively easy to move around if you don't like where you are.
Kontras
Every group at Arm is quite different with different history, culture, resources and management style, but the above applies to all of them.