As the portfolio feature is history in 500px and users are pushed to format.com, here are my first experiences:

1. Format.com offers really great and easy to use templates to create portfolio websites. I preferred the 500px-like template named Amazon to keep the well-known image list gallery style.

2. Format support does not reply to any question via email.
Update: Possibly related to spam issues?

3. Uploading is very slow and does not work for mass-uploads. Image size is limited to below 30MB (not enough in some cases for my Sony Alpha A7RII).
Update: Now > 30MB is possible, upload is still slow.

4. Some template styles are not consistent in font and size for some elements like headlines.
Update: Fixed now.

5. The gallery feature simply does not work for a larger number of items per gallery (via Amazon template): Items are loaded at once, it takes too long time for visitors. This is critical, as it works great in 500px due to lazy loading (while scrolling down).
Update: Fixed now, but still galleries should not have too much images - not a big issue.

6. If one image is opened as popup, after close the position in list is getting lost. Completely unusable for visitors. Update: Completely fixed now.

6. The blog feature is generally great. Thumbnail views in blog item list do not work as expected in the Amazon template. But it automatically takes the first image in blog, which is also great. In some cases it may take some time until the image is shown.

7. Depending on template, some navigation features do not work as expected on some devices / browsers (Windows 10 Surface tablet, drop down navigation does not work).
Still needs verification

Summary: Format has some action items to do before users will have a 500px-like portfolio experience. Update: And they are really working on this! That’s how customer success works…

Update in 12/2017:

Image upload still does not work in Microsoft Edge for some reason. But I’m happy to use Mozilla instead of.