You can now import from PowerPoint and PDF. More info and instructions available here: http://help.slides.com/knowledgebase/articles/487268-import-from-pdf-or-powerpoint
So far this is limited to image-based importing, meaning the inner content of imported slides can’t be edited. We’re hoping to add editable imports at a later point.
An error occurred while saving the commentasmith commented
It looks like the animations don't import from powerpoint. Also, any video or gif in the slides appears as a static image.