The customer service and product support have been really great--everyone we've interacted with over there has been just super nice, friendly and helpful. Also, their searchable knowledgebase ( https://learn.uberflip.com/ ) is quite extensive, and you can usually find how to do most things you're looking for when customizing your content hub--and if you can't find it, the support team will turn around timely solutions for you even when you're crunched.
The backend interface can be confusing at times re: where to find the settings you're looking for, and they sometimes move things around in an update without telling you. The flipbooks were initially very exciting, but when they moved from flash flipbooks to HTML5 a lot of the features / embeddable components we were most interested in using went away. It is VERY expensive for the basic version, and at that pricepoint you don't get API integration (which would allow you to put uberflip content tile on your non-uberflip homepage, for instance) so you are forced to shoehorn the content hub into an iframe or to skin the hub with custom html/css to add your website's header/navigation and make it look to the use like they're still on the same site (but they won't be). I'm not a marketer, but a creative on the implementation side, so the value in all the marketo / salesforce integration is not that apparent to me as tracking leads and calculating ROI is not part of my world, so a lot of the value proposition doesn't strike me as particularly valuable--but knowing which page people stopped reading a certain document on, or other detailed consumption patterns like that would be, I assume, useful to that side of the house.