I bought this mobile about a month ago and I must say I am really impressed with this phone, I believe it is time to share my thought on this phone to the world.
You can see Nokia Lumia 520 features and specifications from nokia.com so I am not going to explain its detailed features; instead, I am going to explain specific features, performance or quality of Lumia 520.
Design & Build: Very stylish and slim 4 inch display. It really looks far costlier than its price range. Feels nice in hand. If you want to buy a stylish smart phone, it wont disappoint you. You can choose one among Black, White, Yellow, Cyan, Red back covers, mine is the yellow one and I believe this one is the most good looking.
Performance: The phone performs smoothly in any situation thanks to its 1 GHz. dual core processor, I always listen to music while surfing internet through wifi from my netbook as well as installing apps and games and it still performs smoothly. Cant ask for better than that.
Some people say it has 512 Mb RAM that is not good enough but I have played games like asphalt 7 (512 Mb game) and it works smoothly with incredible graphics.
Camera Quality: The 5 MP camera of Lumia 520 takes descent snaps and it has touch focus and auto focus. I really love the touch focus feature. The macro mode and panorama is excellent. Though it does not have flash, the night scene mode is also capable of taking clear photos in low light situations. There are lots of applications with very interesting photo editing features. There is an inbuilt photo editor with which you can crop, rotate and auto-fix the images.
Nokia Lumia 520 captures video in 720p @30 FPS and its quality is awesome.

Nokia Lumia 520 macro shot taken in low/medium light

Nokia Lumia 520 panorama shot taken in medium light
Music & video: The sound of the loud speaker is loud enough but it is located at the back portion of the set, so if you keep it inside your pocket the speaker gets completely blocked sometimes. If you do not keep the vibration turned on, you cant here an incoming call. On the other hand, some people are saying that the head phone provided with this handset quality is poor, but the headphone is clear and loud enough and provides good bass.
Nokia Music is incredible, it is very organized and you can find your favorite songs in no times among thousands of songs. You can download songs of your choice for free through radio mix and free tracks of your favorite artists are just a few clicks away.
You can keep your favorite HD videos in the SD card and enjoy them fully on the 4 inch display of Lumia 520. I watched high quality mp4 and avi videos and they ran smoothly.
Touch & Display: It comes with super sensitive IPS LCD (235 ppi) that is unthinkable in this price range. SO, of course touch and display is awesome. Gaming experience in this mobile is magical.
Internet Surfing: You can connect to a wifi network and you can use your phone’s internet sharing option to share your phone’s internet data through wifi. You can connect upto 8 devices. You can get best possible speed from your 2G or 3G connection. I could not find any way to surf internet from phone to pc through data cable.
GPS: It works magically fast in my device. There are many useful GPS related apps which could be downloaded from the app store.
Apps & games: It is very easy to download and install apps and games in windows phone from store. Though windows 08 mobile os does not have as many apps and android, still you can find almost all useful apps in the app store. There are many incredible games though most of them are not free, you can use the trial versions of those games. Skydrive provides 8 GB free cloud storage which is really useful.
Data back up: There is a desktop app for windows to back up your mobile data like images, videos etc in your pc but that is not very user friendly I must say. Of course you can back up your data in skydrive very easily but I can not afford such huge amount of data usage.
No file manager: There is no file manager in windows phone os so you can not browse your SD card or phone memory data directly from your phone. But that does not concern me a lot because all your data could be explored from relevant places, for example, you can browse your SD card pdf files directly from pdf reader.
Battery back up: It provides descent battery back up keeping in mind that it is a smart phone with many features which could drain your battery life in no times.
Overall, no other handset is as good as Nokia Lumia 520 in its price range, that is for sure. It is a value for money product indeed.
I will be updating this post with new points regularly.