Frederik sums it up pretty good in his blogpost so go ahead and read that first.
I had looked at 500px a couple of months ago but at the time I didn't know anyone else on their so I didn't pay too much attention to it. Then last month, right when Fred blogged about it, I arrived at the site again while on some random browsing quest and I made an account.
500px offers a clean layout, a nice portfolio feature and absolutely top notch images (vs Flickr, which hasn't changed at thing in ages and is getting kinda spammy in the comments). So much so that during the first days, I was a little afraid to post my own stuff because everything else on there was so good (read: better than my own work). But I quickly got over that and started adding some images to my profile.
A big 500px-feature, for me, is the portfolio part. I've been planning and attempting to set up a portfolio site for a couple of months now, and either I keep putting it off or the result just isn't what I want. And with 500px my portfolio becomes integrated in my online photo collection, requiring little to no effort to keep it up to date and to add new images.
With that, I plan to finally do something with my janhenckens.com domain. Maybe put the portfolio straight up there, maybe a landing page with some links and a short about page.
500px is not meant to be a dumping ground for every picture I make (for me that is), but it'll be a selection for the shots I like the most (from all occasions, so not just live music).
My Flickr Pro account expires at the end of August and I don't think I'm going to renew it.
You can take a look at my profile here and if you have 500px profile yourself, feel free to follow me :)