Pros
ADTRAN is a stable engineering-driven company, and it's staffed with professional, sane, reasonable people with normal interests and families and lives. If you live in the Huntsville area you can probably appreciate that. People are exceptionally helpful and friendly, and the facilities are beautiful. Everyone gets their own office with a view. The campus is large with many beautifully manicured acres and a large lake surrounded by the two towers. Even the cafeteria serves exceptional food at very fair prices. The pay is average for the area--good for the South, not quite as rich as the defense contractors pay.
Cons
In order to attain promotions you must be driven and committed to excellence, meet deadlines, and at some level you are responsible for making yourself visible to your management. Not that being driven to excellence is a con at all, but ADTRAN is large enough that visibility can be an issue. The management doesn't focus too much on formalisms, and as such, the review process can be lacking. Feedback is hard to come by unless you interact regularly with peers and your management.