I use both 500px
, and Flickr
Both have positives and negatives. 500px generally looks nicer, and cleaner. However, 500px comments seem to be mostly made to get attention - and the down-vote system seems to be harming the ranking algorithm - since less good stuff seems to be marking it to the front page. Having said that the ranking algorithm still works far far better than Flickr's "explore".
Flickr is great for groups, storing full resolution copies of images (worst cast backups), private photos, and it has a great API for integration into other sites. The comments you get on Flickr are generally better, but it's harder to get noticed.
Also maybe this is just me, but 500px seems to favour colourful saturated images of landscapes and women, and cares less about more subtle "artistic" photos. Where as you can find groups for those kind of photos on Flickr fairly easily.
I've taken to using Flickr for general photo-sharing, and 500px for my better portfolio level images.