Hey i updated my Nokia L520 to GDR3, I Will provide update details, for those who are thinking about upgrading can go about it...
First I created a App Studio account, which took me hardly a minute or two,
Afterwards I got the E-mail, clicked on the link to install the certificate,
Then i Installed the Preview for developers application, went thru signing in with my Authenticator enabled live account, agreed with terms and conditions, and enabled the option for getting the update.
Afterwards went thru settings, phone update, checked for updates, and It started downloading, took 3-4 mintes to download(Just a 40 MB update for my Lumia), and 8-10 minutes, for preparing to install(Took another approximately 750MB for this).
My battery was at 29% only, when it asked to install the three updates, I clicked Install, It took 20-25 minutes for all the steps to complete.
So when "update complete" message appeared, I checked with my battery it was at 15 percent...
I had an awesome experience updating my Lumia, just took around 45 minutes from the start, and now I am happy with it, although rotation lock is not working properly, but I feel my phone is more responsive than it was earlier...
Last but not the least, i heard updating thru developer program voids warranty, but only If you are stuck in the process, it will cause trouble for you, otherwise once the nokia official bitter sweet shimmer arrives, your phone will be in warranty again.....
And even If you are stuck in spinning gears, You can use the widely available NSU for Retail app, or the nokia care suite, whichever suits you better....