I want use knockoutjs in a SharePoint's App. I never use it, there are some problem to use it in a new SharePoint 2013 app?
There aren't any problems that I know of. Off course that depends on the type of Apps you are thinking to deploy.
Anyway you just need to add the js reference on your project and use it as a normal ASP.NET page.
For more information you can read this post: http://salvatoredifaziosharepoint.blogspot.co.uk/2013/02/sharepoint-2013-meet-knockoutjs.html