I use it, although I've slowed down on it a lot recently and need to get some pictures up that are more recent.
I like it because, as Patriot mentions, there seem to be more real QUALITY photos than on sites like Flickr. Of course, that was more true back in its early days--I've seen more and more of the same flickr-type stuff as it's gotten more popular. But I still use personally for ONLY my very best shots--everything else goes to flickr.
For me, personally, the one downside is the way the rating system works. The more a picture is "liked" and the faster it is liked (i.e., 10 likes soon after uploading is way better than 10 likes a week later), the higher the rating it gets. A high enough rating moves it to the "Upcoming" gallery or Popular category. The Popular category--THOSE are the truly stunning shots!
My problem though isn't really with HOW the rating system works--it's that I am a little OCD and I'm also a "like"-monger, so if I see that a picture is starting to take off, I will absolutely obsess about helping it on its way. I'll spend HOURS looking through the other pictures, liking things (only if I truly DO love them) and hoping they in turn go look at my photos and "like" them. I've had two photos, I think, hit the Upcoming list, but never had one make it to Popular and can't imagine I ever will.
I have used the portfolio on it to send to a few people who have asked me if I have an "official" website for my photography. Like you, it's not a business for me. I've never sold anything from 500px--I do have the paid "Awesome" account, but I've never really even checked into whether it offers selling options. I'm too busy trying to get "likes" for my photos to worry about getting PAID for them. :lmao: