I have 4 of these Nokia Lumia 510 handsets. Out of which 1 malfunctioned in first year itself and 3 are still working. It was an entry level phone so I was not expecting much from it anyways. The screen was big enough for its time and the camera clicks decent pics.The specifications are not note worthy or anything remarkable.
Pros:
The build quality is sturdy and once you put a cover, nothing can hurt it.
Has Nokia brand name so you can trust it.
It had the novelty factor of Windows phone when I had got it.
Big screen
Cons:
It has no flash or front camera so that is one restriction you cannot ignore.
Not enough ram or no expandable memory. The inbuilt memory is just not enough
Windows phone Os in it cannot be upgraded anymore. It stopped at v 7.8.
If you want to buy a phone today then there are much cheaper and better options available in market.