It'll probably improve, Balham was a slum 20 years ago and now it's gentrified, same with Clapham. I live in Carshalton / wallington and can see signs of my area going upmarket (not least the house prices) so I imagine Croydon will get better.
Being near a tube station adds a lot to the value of the property. Croydon may have decent train links but it's considered 'off the grid' by a lot of Londoners.