I myself purchased the Lumia 520 yesterday, and im pretty pleased with it for a $100 phone.
The build quality isn't as nice as the 900s, but its still quite solid, I actually prefer the interchangeable backs so I have the option to change its color at some point.
The camera isnt as good, but it isnt bad for an entry level phone.
I do miss the gorilla glass, but it isn't THAT noticeable, unless of course I drop it or something, the super sensitive screen is pretty awesome though.
The lack of clearblack display is a LOT more noticeable however, colors dont nearly pop as much as the do on the 900, and blacks arent even close to as good as the 900s.
I dont notice any resolution differences, as far as I know theyre both 800x480 so it looks just as good to me in that regard.
I really do like the 4" screen size, ive never liked bigger phones.
The 520 for me is serving its purpose, I bought it to test out Windows Phone 8, and so far some of the things that bugged me in 7 have been fixed in 8, so my next upgrade is gonna be an even harder choice now between an iPhone and a Nokia phone. Im hoping for another new Pureview windows phone by then since the 1020 was just amazing when I was playing with it in the Microsoft store.