I would like to add the importance of the 7700 squawk. That, more than anything gets every controller's head up whether you're working that sector or not.
I remember one time working a BA diversion into Manchester that made a rapid descent due to a cracked cockpit window. My filters are usually set around FL245, and it was above FL310 from what I recall. As soon as the 7700 came on I could see it on the radar and seeing the altitude drop was the first I knew it was likely coming my way. Gave me chance to get a few aircraft out the way before I got the call from the sector above telling me it was coming.