Fantastic cheers mate, I really hope I am able to leave work early tomorrow to get a decent ride in.
I can't comment specifically on the 520 as I have a 510, but for me it allows you much more freedom to ride roads you probably wouldn't know existed otherwise, without the faff of maps. I map a ride on the laptop using Ride With GPS (easypeasy to do - uses google maps as it's base and for me is less 'fussy' than strava), upload to the Garmin and you're away. I am generally crap with tech so sometimes it will tell me when to turn, sometimes not. But that's unnecessary anyway as you just follow a breadcrumb trail with a very basic map. I think I have taken about two wrong turns in 10000 miles. You can also race a virtual partner as well as yourself.
It also chucks out more data than you could wish for, especially when paired with HR/cadence/power meters. I have the HR and cadence but generally only use the HR meter which is handy to give you a sense of your training zones. After a while I found though that you can pretty much tell what your approximate HR is depending upon how you feel

.