500px started out as a flickr competitor, and like flickr, originally a Canadian company, that morphed into an Instagram competitor with the introduction of their mobile app. Sporting a clean modern look with the usual 'likes' model i thought this might be a good alternative to my long standing flickr account when that platform started to wane under the Instagram onslaught and lack of investment and focus under Yahoo.
500px does indeed have in its favor a modern design where everything is presented within a very clean format that's mostly easy to use. This is actually its best feature.
Where 500px falls down is in its algorithms for promoting new images. For the longest time when you posted a new image on the site, if that image didn't get lots of thumbs up within about an hour it would disappear into total obscurity, never again gathering any views. Recently I noticed that they seemed to have tweaked this to allow an image more time to garner views. This is a welcome improvement though it still relies on the rapid accumulation of 'likes' to sustain any sort of momentum.
Where 500px really fails though is as a social platform. While having a groups feature much like flickr, that feature goes largely unused and unloved. There's very little activity within these groups and most of what is there falls under people asking if their photo is any good, which isn't particularly interesting or useful.
Also it's quite clear that, as with instagram, 500px overwhelmingly favors certain types of images, overly dramatic landscapes, nudes or semi-nudes, and flavor of the week techniques like HDR or light painting. There also tends to be little follower engagement.
The ethical angle : 500px was recently acquired by Visual China Group. If you feel as I do about human rights in China this may be a deal breaker for you. It was for me and I promptly deleted my account.
500px has also come under fire for the popularity of nude images on the site, which prompted Apple to remove their app, and which may or may not bother you one way or the other, from the App Store until it was modified.
500px has also suffered from a constantly shifting business model. First free, then a free service with various paid levels, then opening a stock image service, then changing the nature and royalty terms of that service.