I chose Ext JS in 2011 (an early release of version 4 and Ext Designer) and found this Guide to Designing and Building RESTful Web Services with WCF 3.5 extremely helpful in developing the web service in c#.
It was by no means a walk in the park but now that I'm starting my fifth year of using the two (my web services are still fundamentally the same), I'm pretty happy. I'm using Kerberos authentication in a Windows PC environment (IE/Chrome/Firefox), so users don't get prompted for a password (these are corporate apps which are not accessible to the outside world).