I've found some bugs (reported several of them, waiting to hear back from support), but wanted to give others a heads up on what I am seeing.
If you try to drag an element or field on to the form, and it disappears, or didnt get placed like you think it should, often I think the form thinks the element was added (behind the scenes), but the UI doesnt pick up on it.
Once you add these "phantom" elements, the form gets all kinds of borked. It will work for what had been built to that point, but you can no longer import it without receiving errors, often you lose the ability to view/open the properties of individual elements, and even lose the ability to add new elements.
I'm having to do ALOT of rework right now when this is going on
I'm seeing issues too. When I publish my form and then go create a new item, it hasn't actually published. No errors though.. Also the default value for the choice field doesn't seem to work and rules for showing hiding based on a choice value.
I'm going to stick with the classic form for now. Bit of a shame
Also - beware how big the responsive forms are. Workflows have a maximum size (5Mb from memory) that comes from SharePoint itself - if you exceed it, they may not publish, and they certainly won't export/import. Where the classic forms seem to weigh in at about 200Kb each, the responsive forms are more like 800Kb each.
Also be aware for responsive forms on tasks. For at least the Flexi Task with a responsive task it turned out I could change the list item properties and save them!! Where as the classic NF has all properties disabled.