Perhaps you should post your question to the spring forums. You may get some answers here, but the Alfresco forums are now the wrong place for up to date surf discussions.
My answers.
* How is the lurning curve?
IMO similar to any other ui framework.
* Is SURF already stable?
Its stable enough.
* Is it designed for AJAX-based navigations?
Yes.
* Would you advise using SURF now?
Clearly Alfresco is successfully making use of SURF and it seems to be going well.