They change their special exhibits, but the "Window" themed exhibition on the ground floor was an…read moreinteresting collection of work. Upstairs is the lasting collection with works from Picasso and Matisse to Richter. So a wide array of more "modern" art. Despite the great collection, it wasn't crowded at all, which really helped enjoying the artwork.
It's not a huge museum, so if you have a few days in Dusseldorf, check this out while you're doing the Old Town. It'll only take a few hours.
(Tip: Pictures are not allowed unless you sign a form at the front at get a sticker showing you can take pictures. It's barely an inconvenience.)