I like working at coffee shops because the hum is relatively constant and comes from all around. At an open office plan, in my experience (which is mostly non-mixed: all developers, no sales people), it'll be relatively quiet until someone's phone rings (and invariably they're not at their desk and it rings for a painful 30 seconds)/someone takes a phone call/a group starts talking or joking around. Since there's only one single point of noise, I find it much more distracting than if the entire area was mildly noisy the entire time.